Friday, 18 May 2018

The Black Art of Duplexers: Demystifying Cavity Filters

The Black Art of Duplexers: Demystifying Cavity Filters

I prepared a presentation for the Wireless Institute of Australia (WIA) annual general meeting practical day on cavity filters and duplexers. Others may find it useful. Links to the files at at the end.


A practical guide to the black art of cavity filters for repeaters and digital TV transmitters. Cavity filters are a mystery to most, but at a practical level, not that hard to make or tune. Drew has developed an inexpensive way of building filters using common materials.There are three basic types of filter, pass-band, pass-reject and notch. It is down to the coupling design, which again can be modified or home built. Each has particular applications. All three can be used in repeater duplexers at VHF and UHF. Notch can be used to block unwanted signals like paging TX. Drew will demonstrate a unique use for notch cavities for cleaning the spurious skirts of DVB-T television power amplifiers and the use of low cost software defined radios (SDR) as test instruments for general use and cavity/duplexer tuning. Finally, low cost software defined radios (SDR) and noise sources as spectrum analysers and "tracking generators" for general use and tuning cavities or duplexers is noted.


Cavity filters

  • How they work
  • Different types

  • How they work
  • Cables
  • Tuning
  • Homebrew
Improvised instrumentation

Tuning repeater front-ends

Other uses of cavity filters
  • Filter for DVB-T TX artefacts
  • Notch nuisance signals: pagers
  • High Q filter for very weak signals
Further information

PDF of Power Point presentation:

Power Point if someone wants to use it:

Saturday, 5 May 2018

Earth Moon Earth communications

Earth Moon Earth communications- very draft


Josh, a new call, VK4JNA, suggested doing EME. I quickly went from uncertain to keen as it is very similar to satellite tracking, a long term interest. This post is on EME thoughts, mainly as a record of what we have found; I have a bad memory. It will be updated as we proceed. There will be spin-off posts on things of particular interest.

Antenna systems

Mast and rotators

Locate the antenna at the club; keep partner and neighbours happy.

The club has a new tilt-mast for UHF/VHF. Needs to be erected, about $600 to drill hole and concrete. A priority to get foundation and mount done as the concrete takes a month to cure.

The club has a new G-5500 Yaesu EL/AZ rotator. It needs the AL and EZ rotators to be mounted separately to suit the carriage of the mast.

The club has a computer rotator controller.

I also have the same mast, two of the EL/AZ rotators and a EA4TX controller. The mast base is in, but I need to erect the mast. I have the controller working with the rotators. I was building them for satellite tracking.

While EME and satellite tracking can be done at ground level, having them on a mast gets them above trees, buildings and other obstacles for a 360 degree view of the sky. A view to the horizon is quite important in EME as that is when most QSOs are done.


The antenna depends on the bands used. For 70 cm, a Yagi array is the norm. At 23 cm a dish can be used. Could use dish on 70cm as there designs for antenna and feedhorns.

Yagi array

When I first got back in radio, I researched building high-performance 2m and 70cm Yagi for satellite tracking. I have all of the components to build both, I was following the design in the ARRL handbook that uses insulated elements for low noise. The antenna uses 25mm square section boom, nylon top hat washers and push-nuts to secure the elements.

For satellite tracking, the Yagi can have have both vertical and horizontal polarisation on the same boom, with phasing to get circular polarisation. The small satellites tumble, so both polarisations are necessary.

I had two 2400mm fibreglass poles made locally at $80 each. They were to mount two dual polarisation Yagi for 2m and 70cm. The satellites are repeaters with different band for up and down. The poles simplify mounting as they are insulators and do not interfere with the antenna, allowing centre of gravity mounting. Such mounting reduces the load on the EL rotator.

For EME, the same poles could be used to create the H to mount four Yagi. The EME Yagi are usually single polarisation and easier to make, even though the returning signal has mixed polarisation. I may need to get some more antenna components, but they are not expensive, less than $100. Priority to make first Yagi. Not sure why they don't have better reflectors to stop noise, but front to back ratio is 30 db. Apparently ground effect, but this seems odd if QSOs are near horizontal. Important for echo testing, with the moon viable during the day. Probably a first objective. shop 30mm above boom clamps clamps 1/4" $1.20 each + GST Telrad- telescope aiming device

I have a 2m Yagi with insulated elements and a double T match. It is mechanically easy to make.

Comments from Moon-Net email: re 12.5 Ohm Yagi
DG7YBN 50 Ohm insulated
200...300 OHm-technology (50...75 x 4= approx 200...300).
This is one of the correct solutions for 432 at the small stack:
This is the BG's solution:
Note that Jan, DL9KR works only by CW.
World record of Jan: more 1000 initials in CW mode only.

Also, DL7APV's solution with the open feed line technology:

Yagi stacking

Stacking and power divider design in coax or aluminium square section.

Coax/Heliax- less connectors

Square coaxial- ratio of outer and inner diameters. copper, brass or aluminium rod. comparing 70cm Yagi coax and aluminium QST article

Parabolic Dish antenna

I used to play with satellite TV as I am into home theatre. Prime focus C band 2300mm dishes are cheap new, about $250, but are also available for about $100 used on Gumtree. New is probably easier as collecting used ones are a hassle.

The dishes use a polar mount and can track the geo-stationary TV satellites by rotating in one plane using a linear actuator. The actuators have a reed switch so a controller can follow the tracking.

For EME on 23cm, the dishes need full EL/AZ control. That might be possible with linear actuators and a small computer control, but I am not aware of one (but haven't looked either). It might be possible to use the EA4TX controller, although it uses a variable voltage for position; all rotators use this.

It may be possible to use a Yeasu EL/AZ rotator with the EA4TX controller. The dish would need to be mounted at ground level to reduce wind loads, but the weight of the dish itself is not high. That would simplify mounting and control.

There are designs for the 23cm feed horn and antenna around. They can be made from sheet copper, which I have. The feed horn is to reduce terrestrial noise.

Probably easier to do Yagi array first then try dish. Put dish mount pole in when mast foundation done- Priority


We have 1/4" Heliax, possibly other sizes may be available at the club. -2.7db/30m (1/2" -1.6db/30m). For phasing lines.

For main cable either coax or open line. Coax easier and after preamp(s) and cavity filter. Will check on open line (shielding?)

Coax cable is the greatest loss in the RX/TC system. 50 Ohm RG142 is commonly used. So is RG814. The club has two 100m roles of RG214. It is thick and suited to long runs. We may need to get some more RG142, although I have some. The main advantage of these cables is the shielding to keep extraneous noise out; their losses are similar to common RG213.

Another cheap alternative is 75 Ohm satellite TV coax. It is quad shielded and low loss as it is designed for the first IF of satellite RX from 1 to 2 GHz. It could be used for TX up to 100W pulsed, provided appropriate matching is used.

The alternative is to use separate RX and TX feed to the antenna. This is almost necessary anyway as there is a low noise amplifier at the antenna for RX.

The impedance of coax is different between RX and TX. 75 Ohm is best for RX. About 38 Ohm is ideal for TX. 50 Ohm is a compromise for both RX and TX.

LDF4-50A, HELIAX® Low Density Foam Coaxial Cable, corrugated copper, 1/2 in

Cavity filter

I haven't seen a cavity filter used in EME, but few know about them. It could be quite desirable to have a pass band cavity filter very close to the antenna and before the broadband low noise amplifier. (Not all LNAs are broadband, some have filtering. Filter=losses?)

The LNAs are usually mounted at the antenna, but the loss of a couple of metres of coax (-0.3 db) to a cavity filter (-0.5 db) may be justified to give 30 db of out of band noise attenuation. This would greatly help in reducing overload of the LNA by such noise. With a filter it may/should be possible to use much more gain at the LNA. (and perhaps reducing the need for gain in the antenna?? but need antenna gain for TX!)

QSOs are usually done when the moon is near the horizon. As such, the antenna will receive a lot of terrestrial noise. The cavity filter will reduce some of such noise.

A high Q 70cm pass band filter is a little taller than a quarter wave length, 250mm, and up to 200mm diameter to give high selectivity. The same cavity can be tuned to its third harmonic to work on 23cm with high Q and selectivity.

It may be better to use a pass-reject filter as the sharpness is much higher than pass-band. The reject frequency is immaterial in this application. (No- tried it and pass-band better. Some LNA use small pass-band cavity)

The use of a cavity filter or two might be our contribution to EME practice. In a quick Google I found a discussion about using them but none on actually using them.

Many LNAs, especially high UHF have filters, stripline, sometimes discrete components.

LNA (Low noise amplifier)

LNAs used to be a major problem and a significant cost, however with cell phones and satellite TV, the cost has reduced dramatically. LNAs with a noise figure of 0.5 db are available for about $20.

We need a good source for a LNA with a filter if possible. (Priority) Ordered- L432LNA Down East Microwave. clear photo for cavity LNA. fhx35lg - cavity LNA circuit
FHX35LG- as used in cavity LNA- ordered 5 for $10 obsolete
ATF-10136- lower noise- ordered 5 for $10 obsolete
ATF-35143- even lower noise but surface mount

RX/TX switching

I have some BNC RX/TX relays plus the club has some too.


The club has an IC-9100 with 23cm. I have a IC-7100 to 70cm. IC-9100- 70cm 75W, 23cm 10W. IC-7100- 70cm 35W. Power might be enough for IC-9100, but will need amplifier for 70cm.

I think interfaces are similar. Both probably need firmware upgrade.

Could use 70cm transverter with IC-7300.

I have ic-7300 and numerous SDR RX and TRX. LimeSDR TRX with SDRangel might be good.

I can put a panadaptor on the IC-9100 or the IC-7100. They can be done either at the IF or RF after bandpass filters.


WSJT-X. It is what I had in mind with the GPS RX DSP approach. The signal is sub-audible but can be dug out of the noise. It seems to have its own spectrum and waterfall, negating some of the benefit of the SDRs.

Getting WSJT-X to work on our TRX is a priority.


70cm or 23cm seem the easier to try first. 70cm is probably preferred as we have the most gear for it and the components are't too small.

TX amplifier

The TX output of the IC-9100 may be enough. 100 W of USB Data?

My DATV amplifier is on 70cm and can put out more than the legal limit, although I do need to finish building it.


Moon tracking seems achievable. Probably best to get echo working first. I think it can be done with the moon in any visible position.

Then try listening for signals, then try for QSO. I think this requires the moon to be in a particular place.

EME links

Basics Intro to EME

General Links to all EME. It has papers and links to other sources for EME.

Hardware Minimum configuration. mainly satellite but good. Ideas for dish mounts etc.


Software  Quick Guide To WSJT-X On The Icom IC-7100

Cavity filters for Earth Moon Earth communications

Cavity filters for Earth Moon Earth communications- working draft


Re-inventing wheel? Been done, but not well explained or understood.

Google "Cavity filter LNA" or "cavity LNA" Good. Impedance matching AGO design detail

Cavity filter for EME

I haven't seen a cavity filter used in EME, but few know about them. It could be quite desirable to have a pass band cavity filter very close to the antenna and before the broadband low noise amplifier.

The LNAs are usually mounted at the antenna, but the loss of a couple of metres of coax (>-0.3 db?) to a cavity filter (>-0.5 db?) may be justified to give 30 db of out of band noise attenuation. This would greatly help in reducing overload of the LNA by such noise. With a filter it may/should be possible to use much more gain in the LNA. Further, two cavities could be used in series to give 60 db of noise attenuation.

QSOs are usually done when the moon is near the horizon. As such, the antenna will receive a lot of terrestrial noise. The cavity filter will reduce some of such noise.

A high Q 70cm pass band filter is a little taller than a quarter wave length, 250mm, and up to 200mm diameter to give high selectivity. The same cavity can be tuned to its third harmonic to work on 23cm with high Q and selectivity, and possibly at higher bands.

I tried a pass-reject filter as the selectivity is higher than pass-band, but the out of band dips around the pass, but rises significantly a little way from it, making them unsuitable. The reject frequency is immaterial in this application.

Cavity filter and harmonic response

Cavity filters are essentially a quarter wave antenna, the resonator, in a box, the cavity. At resonance they produce a peak or a notch depending on how they are coupled to a driving signal.

The Q of the filter is very high, but varies with design and quality. The larger the diameter of the cavity, the larger the surface area and a higher Q. With silver plating the Q is increased over aluminium, copper or brass. The design of the coupling also effects Q. However, these are all compromises with cost and other factors. Typically the cavity is aluminium, the resonator silver plated and the coupling a copper loop with an N or BNC connector.

For a pass band filter there are two loop couplings for in and out. The fundamental response is at the quarter wave length of the resonator.

As with most antenna, the resonator is resonate on odd harmonics. This harmonic response can be used to filter at high frequencies, but with the simpler mechanical construction of lower frequencies. I will show how a 2m filter can be used on 70 cm and 23 cm. I can't go to a higher band because of the limit of my spectrum analyser, but this post is to show the general principle.

I had a 2m passband cavity at hand, but not a particularly good one, so results are indicative, but surprising good in this application.

The first photo shows the fundamental response at 145 MHz, better than 30 db, with about 0.5 insertion loss.

The next shows the third harmonic at about 415 MHz, still impressive. The exact multiplier seems to be effected by the cavity's geometry. It is just a matter of re-tuning to the desired frequency.

Even the ninth harmonic on 23 cm is still very good. The insertion losses and artefacts are likely influenced by the coupling design, UHF not N connectors and the cable. Further, I did not normalise the analyser for this shot. Again this is indicative. It would be better to use a 70cm filter on its third harmonic for 23cm and perhaps above.

Cavity filters in EME use.
One effect of a highly resonant antenna, like the magnetic loop of

The cavity filter as used

The 2m cavity filter I used, not the best even for 2m. Pass band filters aren't selective enough for the 2m 600 kHz amateur repeater split. However, they can be enough for 70cm with the wider split. It is possible that this filter came from a 70cm or commercial repeater. I bought four as surplus.

The coupling with its nasty UHF connector. The loop seems small for 2m and may have been used on its third harmonic. However, coupling design is not particularly critical at 2m or 70cm. The earthed long side is close to the resonator. The rotation of the coupling influences selectivity and insertion loss. For high selectivity and higher losses, still about 0.5 db, I adjusted it to be closest to the resonator.

The top of the cavity showing the placement of the coupling loops and the adjustment screw. The rod is invar to minimise temperature changes. However, in this application it is not critical as the peak is quite broad at the resonant frequency. The filter looks very selective in the photos, but I am using a wide span.

Tuesday, 1 May 2018

"Plastic Fantastic" Magnetic Loop- Linear actuator drive

"Plastic Fantastic" Magnetic Loop- Linear actuator drive


I have constructed a linear actuator drive version of VK5JST's trombone capacitor tuned "Plastic Fantastic" magnetic loop antenna for 20 m. The linear actuator drive considerably simplifies the construction and waterproofing.

The linear actuator was too fast at 10 mm/s. A 12V pulse width modulation (PWM) was used to slow it down to a usable speed.

The SWR is dependent on the coupling between the drive loop and the main loop. While 1.1 is meant to be possible, I achieved 1.5 with some adjustment.

Tuning the antenna while operating a SDR TRX is possible as the noise is visibly higher in a spectrum scope waterfall; Win4IcomSuite on IC-7300. The noise on the S meter rises from S2 to S6.

The design

The antenna uses the underground plastic gas pipe now widely available, including Bunnings. The pipe has a PTFE inner, a layer of aluminium then an outer of yellow plastic. With the PTFE inner, 19  mm copper pipe can be used to create a trombone capacitor. See Tregellas 2017 for details of the design.

I have used a 200 mm linear actuator for tuning rather than the geared motor screw drive of the original. These cost little more than the geared motor arrangement, but simplify construction and waterproofing. The cost is about $47 for the 200 mm 12 V actuator.

The linear actuator is positioned so as not to push the trombone capacitor out; they have internal limit switches. However, the actuator will contract too far and needs an external limit switch at maximum capacitance.

The design is further simplified using a plastic chopping board as the main  mount, together with lots of zip ties.

The completed antenna

Detail of the linear actuator drive for the trombone tuning capacitor, mounted on a chopping board.

The coupling loop, which largely determines SWR.

The linear actuator was too fast at 10 mm/s. A 12V pulse width modulation (PWM) was used to slow it down to a usable speed. I used a PWM speed controller from Jaycar at $35. They are available on eBay from about $5 and up.

The SWR is dependent on the coupling between the drive loop and the main loop. While 1.1 is meant to be possible, I achieved 1.5 with some adjustment.


The antenna produces a sharp dip in SWR across the 20 m band with the linear actuator working well.

Tuning the antenna while operating a SDR TRX is possible as the raised noise is visibly higher in a spectrum scope waterfall. I used my IC-7300 with the new Win4IcomSuite.

Unfortunately 20m has been very quiet during the day, just an odd bit of CW.


The "plastic fantastic" magnetic loop antenna works well with a linear actuator, provided its speed is reduced by a PWM speed controller. As a proof of concept, usable RX performance can be obtained, with some room for optimisation. It is a pity that 20m is so quiet at the moment to allow further on-air testing.


Jim Tregellas VK5JST, "The Plastic Fantastic: a Magnetic Loop costing around $54 for 40 metres" Amateur Radio (Australia) Sept 2017 pp 14-17.

VK5KLT paper:


Monday, 2 April 2018

DIY 2m single connector pass reject coupling

DIY 2m single connector pass-reject coupling


In my last post I describe the repair and tuning of a high performance 2 m duplexer that uses an unusual single connector pass-reject coupling. In this post I describe how to make one for about $30 that achieves the same level of performance.

First, I explain what seems to be the theory of the pass-reject coupling as a parallel tuned circuit, using the coax as the inductor and part of the capacitor. The coax conductor is the other part. The variable piston capacitor is in parallel with the coax capacitor to allow tuning. It also gives the necessary mirroring for RX and TX responses.

Then I describe how I made one that achieves about the same performance as the original.

I am very pleased as I have been working on DIY pass-reject couplers for some time. It allows the construction of a complete high performance, six cavity VHF duplexer for about $300, less the cost of connectors and cabling.

Theory of the tuned coupling 

The coupling is a pass-reject type using a parallel tuned circuit, DC from coax connector to earth. Looking at the centre pin, it is connected to the shield of the coax and the conductor is earthed, then the coax is bent into the coupling shape. At the other end, the shield is not terminated but the the conductor is connected to earth. This is a tuned circuit, with the shield both an inductor and part of a capacitor. The capacitor is from the capacitance of the coax at 98 pF per metre. The piston capacitor (1-10 pF) is in parallel with the coupling capacitance, allowing tuning.

The coupling is similar, but the reverse of a magnetic loop antenna. In a magnetic loop antenna, the antenna loop is the tuned circuit and the coupling an un-tuned loop to couple the RX/TX coax to the tuned loop; a transformer. In the cavity filter, both the resonator and coupling are tuned, explaining the pass and the reject. The resonator determines the pass frequency and the coupling the reject frequency, which is how they are tuned. Resonator first for the pass, then the capacitor reject.

The TX coupling is shorter and tunes to a higher frequency, not clear why.

The mirroring comes from the adjustment of the trimmer. more gives RX shape, pass then reject, whereas less gives the TX shape, reject then pass. Not sure why this occurs, some interaction with the resonator I suppose.

The Q of the resonator, cavity and coupling determines the depth of the notch. The Q is high, in the order of 2000 to 5000.

The base of the resonator, effectively a quarter-wave antenna is primarily a magnetic field, whereas the other end, electrical. Being near the base, the coupling is magnetic, as per an air-cored transformer.

Pass-reject couplings are particularly good as they give high out of band rejection as well very sharp pass and reject for RX and TX.

Effective circuit, ignoring resistances.


As with other coupling construction, as I have describe a careful drawing is made of the commercial coupling on grid paper. In this case, I just drew the outline with the coupling sitting flat on the paper. The coax sizes are drawn too big, but my coupling just needs to fit inside the drawing.

The first photo shows my copy and the commercial coupling, the second, the paper tracing.


The biggest problem was getting a minimum 32 mm disk for the base of the coupling. I couldn't think of how to make them; the hole is not in the middle. Then I thought of getting them made on a lathe; expensive. But then I thought laterally. The first was to use large coins that could be soldered. The 1966 Australian 50 cents is 32 x 2 mm and 80% silver; so I ordered some at $10 each. Then I tried searching eBay; stamped brass blanks were available locally at about $1 each; a bit thin at 1.2 mm but 32 mm diameter is ok. They arrived first and I used them.

The coax is semi-rigid RG402 copper tube shied, PTFE and silver/copper/steel inner, available from Element 14 at $20 per m.

I had some piston capistors. They are available on eBay from "element 13" in Bulgaria, or from usual suppliers at high prices. 1-14 pF is ok.


The holes are drilled with a step bit in a drill press. A Dremel or similar is useful for cutting/sanding/stripping.

I extended the centre pin by slipping on a piece of the coax shield to give more length. Bending the coax is easy with the right diameter dowel. Soldered the coax to the pin, cut the other end to length soldered it to the capacitor and pin again.


Checked it would tune to the same frequency as the original and tried it in the cavity; top wasn't perfect fit but it is to go into another cavity anyway. Very close to the original in performance, even with the 1600 MHz split; excellent.

Original coupling


Copy; near identical. Low loss and high reject.

Mirroring from adjusting the trimmer capacitor

Free air resonance


After many attempts at an easy to build pass-reject coupling, finally success. The couplings are for a fairly big tuning range, so dimensions are not too critical.

I will build a copy of the shorter TX coupling and see if it performs in a similar manner. To a point, the one I have made can act as RX or TX by adjusting the trimmer.

Wednesday, 21 March 2018

2m Duplexer unusual design repair tune pager-reject

2m Duplexer design repair tune pager-reject


My local radio club's main 2m duplexer had an intermittent fault. The duplexer was fixed and retuned to a new channel, 1600, not 600 kHz split, partly to move the RX further from a Pager TX.

The Telewave TPRD-1556 Pass-Reject duplexer has an unusual coupler that I had not seen before that works very well, 45 db reject each. They should be possible to DIY. The fault was a defective piston capacitor used to tune a cavity.

An unexpected benefit of the pass-reject RX cavities is a significant 72 db reject of the 148 MHz pager TX. More is possible, if needed from notch cavities at 25 db reject each.

Tuning high performance cavities, 45 db each, highlights the limitations in instrumentation, particularly the dynamic range, 85 db, with the reject response lost in the noise floor.

Design of the Telewave TPRD-1556 Pass-Reject duplexer 

The Telewave TPRD-1556 Pass-Reject duplexer looks like any other six cavity pass-reject duplexer, except that the cavities only have one coax connector not two.

The mystery is in the design of the coupler. The coupler use semi-rigid coax as capacitors as well as the main part of the coupling. There is a piston capacitor to adjust the spread. The design allows a spread from 400 kHz to over 1600kHz, whereas another conventional pass-reject cavity could only go out to 1100 kHz spread.

The couplings are different lengths in order to mirror the RX-TX responses, something that can be difficult to do. I don't know how the do it, presumably with phasing.

Telewave TPRD-1556 Pass-Reject duplexer

RX coupler and detail; ~150 mm long

RX coupler detail showing one end of the centre conductor connected to the coax connector pin but the other earthed. One end of the shield is connected to the capacitor and the other end is connected to the centre pin

TX coupler; ~ 130 mm long

Repair- intermittent piston capacitor

The piston capacitor, while quite reliable, had failed as the ceramic was loose in one end.

Fixing it was simple. It was not possible to just unsolder the capacitor as that would disturb the coupling. I removed the inner piece, then using bolt cutters, crushed the ceramic. Then the bottom portion could be unsoldered and the top part removed. I only had a shorter, smaller value capacitor, but as the screw of the original was most of the way out, I thought it would work, as it did.

New capacitor and original one, showing construction with concentric cylinders.

Tuning the duplexer

Tuning is straight forward with a spectrum analyser and tracking generator. Repeater RX 145.6 MHz, TX 147, 1600 kHz split. The cavities are done one at a time. The length of the resonator is set first and shouldn't need to be readjusted. The spread is then adjusted. The cavities work very well giving 45 db reject and less than 0.5 db loss; within specification per data sheet.

Having done all of one side, I then join two, check, then join the third. The photo shows two cavities, with the reject a bit lost in the noise. The reject should be 90 db, but the analysers dynamic range is only about 85 db. The second photo in each pair is the SWR for three cavities; not sure were the second dip comes in the RX chain, but of no consequence. Between the pairs, the mirroring can be seen.

Two RX cavities showing TX reject, better than 85 db.

SWR of three RX cavities, odd second dip.

Two TX cavities showing RX reject, better than 85 db.

SWR of three TX cavities

Pager TX rejection

The repeater site is near two 2 kW pagers at about 148 MHz. There was a major concern for desensing the repeater's RX. Fortuitously, the club had this faulty pass-reject duplexer that could be fixed and retuned for the site.

An unexpected outcome is that the pass-reject duplexer has about 72 db reject on RX and similar for TX. This can be supplemented by notch filters on the pager frequency to give a further 25 db reject for each cavity, if necessary.

Perhaps obvious, but I was not aware of the potential for pass-reject duplexers to attenuate unwanted signals other than the desired RX and TX for the repeater. This is a significant advantage over other designs, such as notch cavities that reject either RX or TX, but let everything else through.

The repeater's licensed split was changed from the standard 600 kHz to 1600 kHz to further separate the the RX and pager. Ironically, the pager rejection for be 600 kHz split would be 20 db or more, the reject being in the noise and outside the spectrum analyser's dynamic range of about 85 db.

I will report more on this when the repeater is installed. I will probably do another post on this subject as it is an important topic, but incidental to the main purpose of this post.

148 MHz pager rejection for RX 145.6 MHz, 72 db with pass-reject duplexer

Home-made pass-reject cavity filters

I had been experimenting with notch filters for home-made duplexers, very easy to make, whereas I will now try to make pass-reject couplings based on those used in this duplexer. They are particularly good in that they use a single connector , simplifying construction. They seem to be out of patent and should not be a problem copying them, not that I want to sell any. More in another post.

Limitations of instrumentation tuning pass-reject duplexers

Tuning duplexers highlights the limitations in instrumentation, particularly dynamic range. Each cavity has about 45 db reject. Two give about 90 db reject, but the instrument shows noise at the reject frequency, indicating it is outside the instruments dynamic range. All three cavities give about 135 db reject, way down in the noise.

The response can be improved a little using a low resolution band width (RBW), 1 kHz in the last photo. But it takes some time to plot a 10 MHz span at 1 kHz and is only useful for static devices.

I use a Siglent SSA3021X spectrum analyser, a "cheap" Chinese instrument and it has about 85 db of dynamic range. Name-brand ones do maybe 10 db better; not a lot. The specification of the instruments give the noise floor, about 150 db, but don't give the dynamic range. The dynamic range is largely determined by the analogue to digital converter (ADC).

One way of checking the TX reject at the RX is to use the repeater's TX, with the antenna port connected to a dummy load, and measure TX reject signal at the RX port. It is prudent to use a cheap power meter and a variable attenuator to first check that the signal is low enough. The spectrum analyser can then be connected to the RX port and the TX leakage measured; notionally 145 db down on TX output. To a point, the check needs to be done as the repeater's RX is to be connected to the port. The TX reject signal is more indicative than accurate as there can be stray RF from leaky cables and enclosures. Test leads and the repeater cables should be double shielded coax to minimise this.

The other check is the TX output of the repeater and through the duplexer to make sure losses are within expected losses.

I will do another post on this subject as it is important and include some work with wide-band software-defined radios (SDRs) as real-time improvised spectrum analysers.


Friday, 26 January 2018

High power UHF DVB-T amplifier, filters and testing

High power UHF DVB-T amplifier, filters and testing- very draft

I have a used 150 W pallet amplifier from a scrapped DVB-T transmitter via eBay. It is bolted to a heat sink from a satellite transmitter.

I am basically following the 1 kW CW UHF amplifier from W6PQL. I have the low pass filter for the amplifier. I temporarily soldered some SMA connectors to test its frequency response. Down about 1 db at 500 MHz, but -40 db at the third harmonic; very good.

It has been suggested to use a pass band cavity filter duplexer. I had one on hand that I had just tuned for a repeater, trying to get a narrow pass-band.

Just using three cavities, I varied two of the three cavities to try to get a wider response. No joy, little wider, but more importantly 10 db loss.

To be continued...