Re-tuning A UHF Repeater Duplexer Part 1

You may hear members of your local repeater group talking about 're-tuning the cavities'. This is what it looks like.
My camera only records for 20 minutes... I tried to get it done so it would fit on one video but it stopped and immediately started recording again automatically. Sorry about the abrupt ending, part 2 is the continuation of the recording.

[@clintoncann4390]

hi steve this is vk6fcrc great video i'm currently studying for my standard . thanks for the video this has helped in my study of duplexers.

[@TheHellfiremissile]

Great video, just wished you verbalized what the hump measurement was each time. 73

[@ytTaz480]

Actual tuning starts at 12:05. You're welcome.

[@k4iiilink153]

These are simple to tune but tight to their specs, and there are no coupling loop adjustments. This is the second simplest type of duplexer to tune, takes under 30 minutes. The simplest is pass-only, or notch-only (like mobile flat-packs) which takes under 10 minutes. The most difficult duplexers to tune require additional tuning of coupling loops to reach appropriate Insertion Loss and notch values and can take an hour or two if you are picky or looking for specific specifications... TX RX/Bird sells separate models based on your specifications. They have multiple four-cavity 450-470 uhf units like these. 28-70-09 has 1.5dB IL with >100dB notch and 28-70-02 has 0.6dB IL with >85dB notch. You can match both of these specifications on a Telewave TPRD-4544 if you have the proper test equipment and are familiar with tuning coupling loops! I've tuned about 10 and each has over 100dB notches with only 0.5dB IL. Very time consuming though...

[@vk6cs456]

For sure, the repeater group have to modify commercial cavity filters which have adjustments for the tuning loops.

[@bigpimp347]

i understand the large cavities, but what's the purpose of the smaller capacitors, they're a notch right ? but so small are they a tuned circuit inside of sorts and where do these fit in line with the cavities ??

[@jimlanigan7298]

Excellent video. The only thing I wish you would have shown is how adjusting each cavity effects the pass and. I.e. does the left one provide a course and the right one a fine adjustment or are you adjusting both at the same time? Thanks

[@vk6cs456]

Thanks for the feedback. Unfortunately we were running around retuning a number of these so the recording was just an aside as I had a camera with me. All the cavity filters do the same thing, they are all adjusted in the same way. Two of the cavity filters are in the TX path and two are in the RX path. The cavity plungers are adjusted to tune the passband, and the capacitors are adjusted to tune the reject response. So the two cavity filters in the TX path are tuned for TX frequency pass and RX frequency reject, and the two cavity filters in the RX path are tuned for RX frequency pass and TX frequency reject.

[@fdnyfish]

Would of been nice if we could of seen how you tuned it and what you were adjusting.

[@vk6cs456]

Blimey... did you watch the video?

[@srs26]

Hi Steve, great video, could you explain how this works in terms of a circuit, is it a tuned parallel rejection circuit i.e. LC circuit and this is the capacitor? If so, where is the inductor or is it a different type of circuit? Kind regards, Simon M0SYS

[@vk6cs456]

Hi Simon. You may have read this response from part 2 by now but here it is in case you haven't. The plunger is an inductor and the cavity is a capacitor. Varying the length of the plunger inside the cavity tunes the cavity for the narrow passband. Each cavity has a pass and reject setting, the pass is set with the plunger and the reject with the capacitor. The transmitter cavities will be tuned to pass the transmit frequency and reject the receive frequency, and the receiver cavities will be tuned to pass the receiver frequency and reject the transmitter frequency. What I didn't add was a band-pass type cavity filter has a trimmer capacitor across the input and output if you want the notch to be on the low side of the pass frequency, and a fixed inductor with a parallel trimmer capacitor across the input and output ports if you want the notch to be on the high side of the pass frequency.

[@buzzsah]

It seems there's always more to do due to the prev person

[@vk6cs456]

Yes indeed, it certainly doesn't make sense.

[@Etherionix]

If in a thermally stable environment, why would it ever be necessary to re-tune? Assuming the capacitors are air or mica. (except for a change in repeater frequency of course)

[@vk6cs456]

The government demanded a frequency change... many repeaters had to be done. I think I did 12 of these in 2 days.

[@chrisdowland]

I would like to add my 2 cents on the NEW P25-3 systems the typical setup is 2 5 channel combiners that are phased together and the tuning method is tune with a tracking generator bring the system up for an hour then take the system down and wait 15 minutes for the combiner cavities to cool them fine tune once more because it physically changes the combiners internally "very slightly " but once that procedure is completed they never drift unless you change the frequency the digital world is very interesting

[@tiempoder]

Medio video para girar los tachos.

[@nathanjames4242]

Pro tip for people watching: skip everything from 2:00 to 12:00. Its 10 min of nothing.

[@roberts5482]

One mfg issue with cheap crap from China is the plating. Silver will corrode extremely fast. Some connectors use white bronze for this reason. Gold is great for corrosion protection but plated over any yellow metal or copper, the base material will migrate thru the gold and corrode on the surface of the gold. A barrier of nickle has to be applied first before either silver or gold to prevent this. China skips this and uses plating to simulate the appearance of gold but still do not use a barrier to protect it from corrosion or if it is gold the carrot count is very low and is only a couple microns thick. Motorola uses "Hard Gold" which is gold with cobalt added. This helps prevent abrasion to the surface. Gold is the best for corrosion protection but is only applied at 3 to 9 microns thick depending on use. Remember, Silver is the best conductor with copper a close second. Gold is further down the chart of the coef of electrical conductivity. In our game, I prefer white bronze. There is a produce make by Cross Devices called Coax Stuff that will fill voids in coax connectors. I have had some silver connectors I used in them and taped up. After 5 to 6 years, the connectors still looked like new. Another good product is Cortec VCI. I especially like the number 238 spray. It is about $20 to $30 a can. It will keep corrosion to zero in a sealed environment. It can last up to 6 months in an open environment. Bull-frog dot com is the consumer line of Cortec. if you compare the odors of Cross Devices to Cortec, you will notice the same smell. The Cortec 238 is a strong odor like caramel that your wife will not like, so use it outside. I spray the inside of my baluns with it and never have any issues. I've used it in computers in boats and they lasted a long time. We sometimes put electronics in sealed containers filled with oils but the 238 works without the mess. The cortec creates a covalent bond to the surface of metal and will "self heal" if it is scratched. It only takes a little and does not need to be completely soaked with it.

[@robertsimmons1692]

Z