DVB-T TX Filter: 23 cm filter using 70 cm duplexer
I came across a commercial UHF bandpass cavity filter that was as long as a VHF cavity. I removed a coupling to see inside. The probe was nearly 500 mm long, rather than the 170 mm in most UHF filters. They were using the probe as a three quarter length, rather than the typical quarter wave length. I checked the cavity at VHF and, sure enough, there was another peak.
The obvious question then was whether the 70 cm duplexer filter would have another bandpass at three times the frequency, about 23 cm? It does!
With careful tuning it should be possible to use the ubiquitous 70 cm duplexer at 23 cm.
Raw responseWith the filter set to 7 MHz at 446.5 MHz, I had scanned three times that frequency and a very wide span. Sure enough, there was a passband, although at a different frequency and three times wider.
The frequency of these cavity tuners are determined by a fixed length probe (tube) and a capacitive screw adjustment. The relationship between the probe and the adjustment screw is probably affected by frequency.
The bandwidth is three times greater as the upper and lower frequencies are multiplied by three.
TuningWith a bit of tuning, an acceptable 7 MHz filter at 1290 MHz can be achieved. The losses across the channel are a little high but there is usually reserve TX drive. The notches are about -50 dB, which should be adequate as a TX filter.
DiscussionThe general idea of using the filter at three times its normal frequency works. However, the duplexer needs adjust to achieve the desired frequency, bandwidth and notch depth.
I am happy to just measure the performance at the moment. I have a Darko 23 cm amplifier but it is not attached to a heatsink.
It may be better to modify a common 900 MHz duplexer by reducing the length of the tubes, but it is a fair amount of work to pull them apart.
A notch duplexer can be used at three times its nominal frequency as the probes act as three quarter rather than one quarter "antennas".