Cavity filter not working?

I had a HAM guy tune it on his analyser, said it was way out, we went for 10 mhz seperation but its still not working.
He said its quite difficult to filter out with one of these and said you might need more filtering added.
Other option is to use two seperate filters and antennas.

I have since sourced a couple of similar filters, I take it I can use just one half of each of these filters on the TX and RX units?
With the open port on the end, I assume I need to add a 50 ohm terminator?
N connector terminators seem to be jolly expensive, will soldering a 50 ohm resistor across an N connector work just as well?

Have you tried placing the receiving radio in a shielding box? I suspect that even if the duplexer were working perfectly, a UV-5R might be shielded poorly enough to still receive RF through the body of the radio and get desensitized.

Also, if all else fails, you could set them up as a crossband repeater. Most projects I’ve seen using UV-5R radios to construct a repeater either have the receiver on 2 m and the transmitter on 70 cm, or vice versa, with two antennas. Most dual band radios, including the UV-5R, can have a channel programmed to receive on one band and transmit on another for using this repeater.

I wouldn’t recommend using this setup for more than talking between a few friends, but then I wouldn’t recommend using a UV-5R for anything especially important anyways. I say this as a happy UV-5R user, but also one who is aware of their limitations.


OK, i didnt think much about shielding the radio!
Do you think wrapping the radio in tin foil would be enough?
I should give this a try first, that would be an easy fix!

Did the guy who tuned the filter explain why it wasn’t working? 10MHz is quite a decent and workable split - after all, if ham repeaters can cope with tiny splits, and commercial ones with 4.6MHz or more, 10MHz should be fine. My repeater with 453/459 split works fine with a simple multi cavity filter (5 cavities)

He told me it was way out, but was understandable after I tried tweaking it.
He said these filters were “notching” the signal, you could see a spike on the analyser and he tweaked the screws until it showed a nice clean spike at the right band.
It would make sense i guess if these radios are picking up noise from the transmitter in such close proximity.

Filters come in two common types - those that have a high rejection at a certain frequency and let everything else through, or ones that reject everything, bar the frequency you want. Filters usually come in two sections, fed from the common antenna, so you tune them to allow the TX frequency to pass through, but frequencies either side are attentuated and then you tune the other to do the same for the RX frquency. The adjustments for one, mess up the other, so you go back and tweak that, then that messes up the other and so on - eventually, you have a working compromise and with typical commercial distance between the two frequencies, the TX does not desense the incoming recieve frquency, and the incoming signal is not attenuated very much either. On mine, if I have a weak signal coming in, when the TX keys up, the signal gets just a tiny, but noticable, but weaker. That is really the test. can you hear somebody right on the fringes? If they vanish when the transmitter powers up, then they’re too far away. Tuning cavities is a dark art.

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Yeah that sounds about right, he said it was a rejection filter. And your right, tweaking one side does slightly mess up the other side a little.
The issues im having with desense is when someone is speaking to me a few km away, i can hear the signal from his radio quite fine, but through the filter its weak enough that its being cancelled out im assuming.
Actually, this could be a good test for me is to try transmitting on my repeater and place another one of these handhelds nearby and see if i can hear his signal while im transmitting out. That way we can rule out desense in the filter or if shielding on the radio is the issue.

This should be a quick and easy question and the answer might help the OP:
Do these space-efficient filters need to be tuned for specific channels or can one side be tuned for 462.55-462.725 and the other tuned for 467.55-467.725? Maybe someone tuned it for the wrong channel.

Pretty much a single channel - one of the propular Retevis products is a portable repeater with a similar filter in the box - the radio is multi-channel! Pointless because going just one channel up or down, drops the performance sharply.

Single channel on each side, ive tried shielding my radio with tinfoil but it hasnt made any difference.

When I transmit, the receiving signal just goes quiet.
The guy who tuned it for me is pretty clued up, he has alot to do with HAM radio. He hadnt really worked with this type of filter much though.

Well, if the transmitter is more than 5Mhz away from receive, then sadly, the filter hasn’t been tuned properly. Whatever the notch is, it’s just not right.

Well I just had an expert look at the filter and he said these filters typically do the job fine, but turns out this one has an internal short inside, he pulled it apart and it was full of filings from factory that fell out!
Anyway, that improved things after cleaning them out, but one of the cavities is still giving issues.
I had since found some other flatpack units that came out of commercial gear and tuned it for me and said it should totally be OK at 5mhz separation so have settled back on that.
Now im still having issues with this and he tells me with the baofengs its cruical to filter out noise and RF leakage.

He recommended the following, I want to know how critical any of this is:
Add toroids onto the microphone cables that link each unit to the duplexer, he said they will act as an antenna and induce RF into the radio, ive done this but made no difference.

He said that the coax cable length to the cavity filter should be tuned to half the wavelength of the tx or rx frequency each set is working on.
He said the formula is divide 300 by the frequency and then divide by 2 and you get the length in millimetres does this sound right?
Im also using rg58 coax on these jumper leads, he said that will be radiating out RF into each set.
Am told RG402 would be a better choice for this.
Im also using this coax to the aerial itself for testing purposes, dont know how critical this is, but want to use RG213 on the final unit.
Regarding the aerial, he says that the tunable antenna with the internal parts that slide inside each other will affect the impedance and will not give true 50 ohms which will affect things.
Not sure if this will make more noise however.
He said to scrap the whole antenna and just make one from a piece of wire, solder it to the centre of an N connector and leave it at that.
He also says that the PL259 connectors affect the impedance on UHF and to stay away from , does this sound right?

Anyway, I want to know if any of these factors are detrimental or not.
I also need to work out of tin foil shielding will be enough around each radio or not, just for testing purposes anyway, I dont want to waste money on precast boxes if they are no use.

OK - reading your post makes a few thing quite clear now. You have had an expert look at things and for some reason you want to find fault? Reading what you typed, I have a very good impression of the technical guy because what you understood is wrong in quite a few areas, but what he said, and meant, is right.

There’s no point trying to go through these things phrase by phrase because you haven’t noticed when in is out and a bit of masic maths. I’m sorry, but you are trying to do something quite complex and set up specific in a very strange way. You’ve not even worked out what he meant, and have misunderstood. I’l look at a couple of misunderstandings. 300 divided by the frequency in MHz gives you the wavelength in Metres - 145MHz as an example is 2.068m - dividing by 2 is a half wavelength, His comment about soldering a piece of wire to the centre pin would require you to0 divide by 4. There is a bit of extra maths to do it really accurately, but you are in the ballpark.

The bit about the tunable antenna with sliding adjustment - he means that the sliding adjustment sets where the antenna is resonant - i.e. performing at it’s optimum, and being 50Ohms. The filter is tuned with a 50Ohm load. Your antenna working on two frquencies cannot be perfect on both. One of his tuned ports or even both may not be seeing 50Ohms - where he tuned it. Small, but filter tuning is tiny tweaks to make magic happen.
PL259 connectors are tough and cheap. They were crazily originally often called UHF, when they’re really not good in that band.They are what are called non-constant impedance connectors, N type, BNC and TNC are constant impedence designs. PL259 connectors change their impedance depending on whatfrequency goes through them. Again, very small amounts and often ignorable - but the statement is true.
Torroids try to prevent the RF in the air locally, from being ‘captured’ by the mic cable (like an antenna) and sent along the cable to the radio where they can wreak havoc. They present a high impedance (a barrier) to the RF, but don’t impact audio. In general, small ones reject RF a teeny bit. Huge great ons reject more.

Coax cable can shield the internal conductor 100% at best - things like heliax - a corrogated bendable outer wrapper, or modern foil screen flexible cables - they wrap a piece of conductive foil around the inner for the same good screening. RG58 and RG8 and loads of others have braid. The amount of copper, or worse, aluminium in the braid produces ‘holes’. In cheap cable when you strip it, you can see the white inner. The RF leaks out through the holes. In fact, they make VERY leaky feeder cables and use them in tunnels, or inside buildings where rebar or metallic wall panels prevent RF getting in or out. The feeder cable allows RF in and out. This is undesirable in a carefully tuned repeater system, so they use cables that don’t leak.

We need to be a bit honest, too. baofeng radios are designed to transmit and receive and be cheap. If you look at the ‘purity’ of what comes out of a decent transmitter, it is power on the desired frequency, with a band width to a careful specification. In FM systems it means the deviation is adjusted to a specific amount. either side of that one frequency there should be nothing. If you put (for discussions sake as I have one on the analyser at the moment) an Icom Marine radio on channel 16, there is a big spike on 156.8MHz. There is a very, very small output at 313.6MHz and an even smaller one at 470.4MHz in the UHF band. These two frequencies, the harmonics, are created in the transmitter and the radio filters them out so I cannot hear them on a radio next door. If you put a Baofeng on the analyser and do the same thing = it isn’t just those two frquencies that are visible, there are dozens of small spikes all over the place. This is why they are cheap. These spurious outputs also annoy the filter. all these spikes interact. The sum or the difference between them can creat all kinds of noise. The Icom marine set when listening on ch 16 does not hear the rubbish that the Baofeng suggests is present. Ch0 - used here for the coastguard is on 156.0MHz and the Baofeng has a solid buzz on this channel, the Icom does not. The buzz is harmonics from the router switch mode power supply under the desk. Icom filters the crud out, Baofeng does not.

There is a reason even Chinese repeaters start at around 800 Dollars or so, without filters. They need better performance, better screening and better design.

Your technical fella trid really hard - why would you disbelieve him? Sadly you also misunderstood him because youre not yet skilled in RF engineering, which he seems to be. You have bought a cheap, brand new go to the shops car, and are fitting go fast exhausts and a race tuned head to it, using off the shelf components and no knowledge of engine tuning - and expecting to enter it in a race? This is a silly thing to do. Your 5MHz split is never going to be stable and efficient with the radios you have and antennas, cables and other things bodged. You could put the receiver in a metal tin, put the transmitter in another and that would help them quite a bit, but they’re really not up to it, design wise.You can at the moment buy portable reepaters in a box with filters inside. They advertise them as multi channel, but while they have that capability radio wise - the filters simply cannot do it.

You are chasing rainbows with this project. You will have short range repeater operation, but desense will mean the transmitter wipes out weak incoming signals, and that really is as good as it will get. Sorry.

I never said I doubted the guy and I dont profess to be a radio engineer, but I know the basics.
Yes I understand now that filtering RF through one of these is not so easy, but this fella assures me these radios will work fine if you do it “right”
Ive spoken to a few radio HAMs and they all tell me different things, I know that all these measures he recommended are good engineering practice, just dont know what ones were too critical, such as the PL259 connector on the antenna, etc.
Im still testing the unit only using the RG58 cable at present, but think it may be a good idea to go to RG213 for testing purposes.
He never instructed me how to calculate the length for the UHF antenna, but I thought he was telling me to make it full wavelength, which is the frequency in millimetres/metres? e.g 465mhz would be about 644mm?

He was telling me to “detune” the coax by making it half wavelength to stop any resonant signal getting out easier into each radio. I dont know enough on the physics involved, but if this will help, I will definitely do this.
It sounds like using better coax will help things so is probably wise to do so, main problem is i was told by another HAM i trust well that it is not cruical.

Yes I am aware of the issues with cheap baofengs, at the time I assumed they may work fine, but they dont have the greatest shielding and im wondering if they are making lots of side noise when transmitting, I could connect it to a spectrum analyser to see if this shows much, Im wondering if i can find a better radio such as a kenwood to use on the transmit side perhaps?

I know about tuning the antenna to get resonant, thats easy, he was telling me that these cheap tunable antennas that slide inside to tune the wavelength will have stray capacitances and affect the impedance, i.e not give you a true 50 ohms, he told me it will cause issues.

Anyway, i was told to try two antennas and that still was giving me issues, no matter how i separated both (vertically and horizontally apart)

I will let you know my progress anyway, but I think im on the right track.

I understand, but you’ve just got a few gaps in your knowledge which prevent proper understanding of his comments. Your maths is correct, but what use is calculating the wavelength? Antenna designs centre on half wavelengths quarter wavelengths and ¾ wavelengths because two of them match well to coax, one doesn’t and needs conversion. The maths also misses out velocity factor components, but are fine normally because the last stage is cut with a meter. The sliding tube designs introduce features that impact on resonance, like capacitance and inductance, but none of these are wrecking your system, it’s not suddenly going to behave when you swap PL259 for N type. You might get better results from putting the two portables up a pole using their own antennas extending the control/audio cables and scrapping the filter? Or putting each radio in a Faraday cage and using full screen cable to antennas widely apart. It sounds like you have antennas that are not 50Ohms but the filters were tuned for 50? This just reduces filter efficiency and isolation of input and output is essential. Swapping a short patch cable might give you a percentage of a dB, swapping to a quarter wave ground plane pair of antennas might do the same thing, but how you mount these on one pole could mess up the system again with impedance changes. There is no short cut to effective repeater installation. Everything you have is a compromise. The radios, the filter, cables and antennas. None are optimum. Out of the people you have asked it seems everyone is really giving you the same advice. I don’t understand why you’re persevering. Does everyone have dual band radios that can work.

Interesting what you say about antenna designs, I was sure that full wave antennas are a thing, particularly with shorter wavelengths.
This is the UHF antenna im using just for reference.
Im pretty sure its full wave, its tuned length measures thereabouts what I would expect for the frequency im operating at.
Retevis MA01 UHF Omnidirectional Base Station Antenna (Link removed - Please read the Forum Rules regarding links to other resellers selling similar products).
If its a waste of time using in my application I will look at something else.
Going by what you say, I dont need to panic about the antenna design or the PL259 connector.

But I was told that going for a simple whip antenna is not quite as easy to build as it sounds to get an accurate impedance and can require lots of tweaking.
Anyway, this one ive got has an internal loading coil and is probably OK, providing its actually been well engineered, It is a chinese brand after all, but its price tag and build quality seem to suggest its not rubbish.
If i seperate the two radios, how far would they need to be apart to avoid issues?
This is something I could look at if necessary.

The antenna above appears to be 50 ohms, and the coax im using is also.
The filter was tuned on a 50 ohm load, so I assume that should be OK.
Plan B could be to switch to a UHF/VHF hybrid system, but I will see how I get on, Ive put alot of time and money into this, so would like to get it to work if I know its possible.

The problem again is you really need a bit of study. It won’t be a full wavelength antenna as a vertical - that wouldn’t work. This is a stacked antenna - probably, because they don’t even give any technical specs.

Ground plane antennas need to have an impedance at the feed point of 50 Ohms. The vertical quarter wavelength type is the obvious one - so around 17cm in length - typically the length portable radios have on them for UHF. gain, over a dipole type? Nothing! Sometimes a teeny bit less because the ground plane design makes it a little less effective. A ¾ wavelength also haS A 50 Ohm impedance. On my work van I have a ¼ wave for VHF - cut for about 150MHz so I can use it on the ham band and on marine band. Not an exact match to 50Ohms at either - but it works fine. It is also a ¾ wave for the UHF ham band and our commercial UHF business band. A simple antenna, not too long, and useful.

They also sell 5/8th wavelength types - these need a coil at the bottom to add electrically some inductive load, and then they perform pretty well. Your antenna is probably two different antennas stacked on top of each other. It’s quite common to see a UHF quarter wave, with a ⅝ antenna sitting on top of it - it creates some gain by the combination. My best guess based on look and length would be a ¾ with a 5/8th sitting on the top? A bit odd, but it could be made to work. Downsides? Tuning. It’s that collar with the allen screw that suggests this allows the element to be tuned - if you have a decent VSWR meter you trust. This really is quite a complicated area of study - antenna theory starts very simply then goes upwards out of my league speedily. The other common antenna type of course is the half wave dipole - half from end to end - a downwards quarter wave and an upwards quarter wave. Commercial types are folded, which increases the impedance and then the balun is designed around the 4:1 types to get the impedance down again. Verticals can be made by a sleeve dipole - the feeder cable goes up, then the quarter wave goes up. The lower quarter wave is a tube that slips over the feeder and goes up to the point where the topic section emerges. Stick it in a fibreglass tube and you have a marine antenna - that will work on a fibreglass boat with no ground plane. All these different modes of operation are not always easy to spot. Lots of antennas may be compromises and have little coils and/or capacitors to adjust the resonance and impedance. If you can see them, great - but your antenna has that black plastic bit in the tube. Is that just a plastic moulding to join two tubes, or is there a coil/capacitor inside it? No way to tell without pulling it apart. A quarter wave vertical is the absolutely simplest antenna you can ever have. A bit of wire poking up from a ground plane. You cut it longer than the maths suggests and measure the VSWR. Let’s say it reads 1.5:1. Maths tells you it is too long and the VSWR is not perfect. You nibble off a quarter of an inch. Test again - the VSWR might drop to 1.4:1 - you are going in the correct direction, so you nibble a bit more, maybe an 1/8th inch next time and watch the VSWR drop. At some point it starts to rise again. That’s it - no more nibbles. ¼ wave antennas are simple, can be made from coat hangers, block connectors - and now Wago connectors. Some of the Wagos have 4 or 5 commoner outlets - so that’s brilliant for making the ground planes.

Every ham used to experiment with this kind of thing - now they buy them and get confused over what they are, like you’ve done. . You’ve discovered the loading coil (just re-read it) so it will be a combination mode antenna. You have put lots of time in - but in repeater world terms, not money. Repeaters on the cheap are nightmares. If you do have crosswind facility, your problems will go away. Pick two non-mathematically related frequencies and have a really good repeater - even with the antennas on the same pole. Years back it was common to have little splitters so a dual band antenna could be plugged into two radios - putting these into crossband mode worked great. The diamond 30 style antennas work really well on crosswind repeat. In band repeat on poor receivers and spurious signal transmitters is a nightmare.

Ask yourself a question. How do you know when your antenna is 50 Ohms at the feed point. DO you have a 50 Ohm dummy load that confirms the VSWR reading? For a long time I used a VHF/UHF VSWR meter and when I started doing commercial radio work I bought a nicer one. My Commercial repeater on the roof here was perfect, match wise, but the new meter said it was 1.2:1 close enough to 1:1 I suppose, but sweeping up and down the band, 1:1 was actually 12MHz higher than I was using Licence for 453, antenna was tuned for 465 - which is way above. No doubt this contributes to the range I have always had which is a similar issue to you. In my case, it covers the area, so it’s just not worth sorting - but your problem is worse, pretty terrible performance. I suppose you could buy an analyser and a VSWR bridge and tune your own cavity on your antenna and spend hours tweaking and honing the performance - but you still have rotten old radios driving it, and they’re great for being portables, just awful at being an in-band repeater.

Do NOT equate quality with price. A turned piece of aluminium with threads is not an issue no matter where it is made. Sliding joints, plastic components and socketry very much is a gamble. One antenna I buy from China has been brilliant for fifteen years. Last delivery there were subtle changes. A hard plastic component is now more like nylon and slightly less rigid. The N type socket was screwed into the base casting - now it is a push fit with single clamping screw, so will it keep the rain out???

OK, that is alot to take in!
I know antennas can be ridiculously complex things.
I do have a 50 ohm dummy load and a SWR meter, its just one of those ditigal surecom meters but it works OK.
I was getting a SWR of 1 when i tuned it right.
I can only trust the advertised load of 50 ohms is right but you need an antenna analyser to test correctly?

I play around with electronics alot, but have only gotten into radio recently. I have built a linear amplifier some years ago to use on the 10 metre band.

If I go down the UHF/VHF route, would one of these work OK? (Link removed - Please read the Forum Rules regarding links to other resellers selling similar products).
What kind of antenna would you recommend? Could I use this same antenna tuned to the UHF band for TX and still receive VHF signals OK?

Those small ones are pretty decent for this as they just filter above and below a spot between the two bands and within their range they have little loss and enough isolation. For cross band repeaters, they do a good job unless you run high powers. Up to 10W or so, no bother.

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If using crossband, what will the lower (unlicnsed?) frequency be? Hope its not on a sub harmonic of the UHF radio.

And why is everyone on amazon, MFJ included, calling those duplexers? They are diplexers.

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