> >A Home Brew 224 MHz Repeater Project. > >Let's talk about the coax connector being an RCA jack on the > >exciter PC Board. Why bother with a box chassis mount connector > >added to the path
> n...@... wrote: > Why? To maintain proper shielding. No, to eliminate the extra loss of two additional jacks and plugs. Since the exciter output is down near the trailing amplifier's practical minimum drive level (value), it's nice to where possible reduce unwanted signal loss. > Again, passing a wire/shield/any conductor through a hole > in the box without bonding at that point will make > the conductor act like a coupling probe. Sure, it may > work but I claim so would doing away with the box > altogether, so you could simply bolt the exciter board > to a plate. Of course the harmonics generated by the > exciter will radiate. I tried and measured at least three different construction techniques... all had the same results. The first was just holes in the box, no bonding to the metal box. Keep in mind the actual length of coax (inside the box) routed to the exciter RCA Jack you assume is an RF probe is really short. The second method is the addition of hole cover plates, which in this case each have holes for the coax to be soldered or compression fit at the box entrance (yes the compression fit makes an electrical connection. I also tried some "V" compression (and solder) plates based on an examples of adjustable coax entrance plates now being sold to/for the Amateur Market. I like and used this method most when the mentioned connector loss is a significant concern. Tres was the traditional chassis mounted N Connector female jack. A decent 50 ohm coax connector is probably the best way to go but you are subject to the jack and plug (connector losses). La final' like the first example was direct RCA plug coax connections with the addition of RF Chokes. I did a lot of "RF looking around" the exciter, at pc board level, near the board, in the box with the board running, box cover off and box cover on. Credit to the latest version of the Hamtronics T-301-6 Exciter for being well thought out in regards to RF shielding and having very minimal unwanted RF products both near-field (radiated) and out the coax port. > If there are any RXs at the site that are harmonically > related to your 220 TX you'll probably find out about it. I found the strongest harmonic from the exciter (by direct connections), inside the box the signal was pretty much undetectable as a radiated signal. Pretty much nothing made it outside the box except the expected signal coming out the coax pipe. > I once had a similar problem in reverse (440 MVP hitting > a co-located 220 RX exactly 1/2 the MVP TX freq.). This > is where I learned about effective shielding techniques, & > why you can't do what you outline below. > > when in many cases it's more loss than it's worth. So... > > I soldered an RCA connector on the end of a section > > of quality small size (brown tan) Teflon coax and routed > > it through a hole Sure you "can" do exactly what I described if the situation allows it. I measured everything inside and outside the box and there are simply no radiated RF problems with the constructed repeater. Your point of the RF probe action from a through chassis lead is valid for a common construction technique, but the actual lead "probe pickup" is dependent on a large number of variables so it is not a problem generator in this case. Even in the first construction example the box outside unwanted RF products from the exciter are simply not getting out of the box. The final repeater product has the RF Shield compression hole covers in place with no measurable change in performance and unwanted radiated RF levels. > > In the case of this model exciter, one can key the entire > > module each transmission knowing the synthesizer requires > > about 350 mS to come on the air. > Yuk! Wonder if there's some way of leaving the synth. > powered up while disabling the rest of the exciter so > you don't have to deal with this delay yet still not > have the TX signal heard near the site. > Bob NO6B Yes, it's mentioned and suggested in the Hamtronics T-301-6 paperwork (manual). The synthesizer power is separated from the trailing RF stages and optionally left powered on 24/7. I chose the key the entire exciter each time layout with an easy outside the box jumper change to keying the trailing RF stages option. The repeater users don't seem to even notice the delay nearly as much as the 30 second time out time. I've constructed and installed a second and now starting a third 224 MHz Repeater. Pictures of said will soon appear in the group photos section with the first 224 Repeater photos. Each project is different from the previous as I use various resources. Repeater number 2 had a Henry RF Amplifier that became unstable regardless of what size whip and chair I used to beat it into submission. I don't seem to receive great results from a lot of VHF and UHF Henry RF Amplifiers. So out it went replaced by a new Mirage unit. I also used a single center divided box, which works out better but also has its own unique set of issues. Both home-brew 224 MHz Repeaters are now installed and working well at a very busy high level repeater site. Onto the third repeater order before moving to the next project. cheers, s.
