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.
Thereâs no point trying to go through these things phrase by phrase because you havenât noticed when in is out and a bit of masic maths. Iâm sorry, but you are trying to do something quite complex and set up specific in a very strange way. Youâve not even worked out what he meant, and have misunderstood. Iâl 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 itâs 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 theyâre 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 âcapturedâ 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 donât 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 âholesâ. 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 donât 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 âpurityâ 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 isnât 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 theyâre 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.