Re: [Repeater-Builder] Re: A Home Brew 224 MHz Repeater Project. - Part 3 (The Exciter)
At 5/24/2009 21:57, you wrote: > > >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. OK. But if you really wanted to minimize the loss between the exciter & PA, you could use an N bulkhead (virtually lossless @ 220 MHz) so you could use some larger diameter coax between the box & PA, or even Superflex. > > 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. OK, that's probably why you don't get much coupling to the outside of the coax. At 440 (where I normally work) & higher frequencies, it's much harder to keep those distances short enough to avoid problems. >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. Is the synthesized output the fundamental signal, or is there multiplication after the oscillator? If it's the former & they're using a clean oscillator, there may be little harmonic content generated on the board. Bob NO6B
[Repeater-Builder] Re: A Home Brew 224 MHz Repeater Project. - Part 3 (The Exciter)
> >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
Re: [Repeater-Builder] Re: A Home Brew 224 MHz Repeater Project. - Part 3 (The Exciter)
At 5/8/2009 21:26, you wrote: >A Home Brew 224 MHz Repeater Project. >May 2009 >Part 3 The exciter (transmitter) >First Post May 2, 2009, Second Post May 6, 2009 - This Post May 8, 2009 > >Moving right along... (snip) >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 Why? To maintain proper shielding. 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. If there are any RXs at the site that are harmonically related to your 220 TX you'll probably find out about it. 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 made just large enough (to pass the RCA plug) with >a Unibit Step Drill (bit). A low cost clone set (three in one >package) of Unibit type step drill bits are almost a must have >expense of less than $15 at Harbor Freight. > (snip) >The disadvantage of leaving the oscillator on all the time is a >constant "local" low level signal that is sometimes a lot stronger >than you would expect. One of my 224 MHz Spectrum transmitter >oscillators can be heard almost a half mile from the repeater >site (which is not really very professional or something to be >proud of). > >In the case of this model exciter, one can key the entire module each >transmission knowing the synthesizer requires about 350 mS (Milli >seconds) 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
[Repeater-Builder] Re: A Home Brew 224 MHz Repeater Project. - Part 3 (The Exciter)
A Home Brew 224 MHz Repeater Project. May 2009 Part 3 The exciter (transmitter) First Post May 2, 2009, Second Post May 6, 2009 - This Post May 8, 2009 Moving right along... This text is part 3 of a description of a recently completed 224 MHz Repeater Project. One could easily apply the same techniques toward a repeater project in different frequency ranges. Pictures of the completed repeater project reside in the group photos section. http://groups.yahoo.com/group/Repeater-Builder/photos/album/1157128983/pic/list I selected a Hamtronics T301-6 exciter (transmitter) for the project for some of the following reasons. The exciter is frequency synthesized, relatively low in cost (about $229 each at the time of this post), respectable in performance and fairly straight forward to interface. Hamtronics normally requests the frequency of operation at the time of purchase so the receiver arrives "pre- tuned" and ready to interface after mounting. Unlike the receiver portion of this project I did not have an available original Hamtronics Equipment box. I did have a basic LMB type Aluminum box with a matching cover, its dimension about 7x5 inches. I drilled the proper locations for 3/8 inch threaded standoffs at the PC-Board mounting holes. There are no external controls so one only need be concerned with the RF Pipe (coax) and wire connections normally made through the rear panel. Be sure to pay attention to the box height requirement as the coil forms can "stick up" quite a bit. I've used 2.5 inch high boxes but the luxury of a 3 inch height box assures every coil form should easily clear the box cover lid. I again chose the feed-through capacitor method for routing the power, logic and audio type connections through the box wall (rear panel). The picture(s) should give a fair example of the same 3X3 hole drill pattern I used for both the receiver and transmitter FT Caps. Also note the solder ground lugs used mounted with some of the capacitors on both sides of the box wall panel. 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 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 made just large enough (to pass the RCA plug) with a Unibit Step Drill (bit). A low cost clone set (three in one package) of Unibit type step drill bits are almost a must have expense of less than $15 at Harbor Freight. The other end of the Mini Teflon Coax routes directly out of the box to the RF Amplifier mounted right next door. My only grief was trying to find the original one piece RCA plug. Everyone wants to sell the two-piece plug more suited toward the audio crowd. I used one from the salvage bin while the replacement parts RCA Plug stock back-order gets sorted out. CTCSS encoder audio is routed from the TS-32 in the receiver box to the proper feed through capacitor, the wire on the box inside makes the connection to the proper point on the exciter. All the connection points are clearly described in the Hamtronics Manual. The output of the TS-32 has a lot of available voltage so a knock down series resistor (like say 15K) might put the level adjustment pot up off the almost off - zero position. I didn't bother but I will say setting the CTCSS injection level is something to do with great care. Hamtronics like a number of other Manufactures of current and previous transmitter modules provide a choice of two methods to "key the exciter" RF on and off. A classic method is to operate the low level oscillator section and switch supply voltage to the trailing higher level RF stages. This allows the oscillator or synthesizer to run constantly, which in many examples place the transmitter on the air fairly fast. The disadvantage of leaving the oscillator on all the time is a constant "local" low level signal that is sometimes a lot stronger than you would expect. One of my 224 MHz Spectrum transmitter oscillators can be heard almost a half mile from the repeater site (which is not really very professional or something to be proud of). In the case of this model exciter, one can key the entire module each transmission knowing the synthesizer requires about 350 mS (Milli seconds) to come on the air. There is a provision on the exciter module to delay the RF until the synthesizer is stable and proving a valid lock signal so you're not sweeping a signal across the band as you power (key) the exciter each transmission. So I tied both the synthesizer and RF Chain supply leads together and switched both to the supply voltage with an active low logic made using a small 12vdc reed relay sold by Radio Shack. You can see the relay glued to the chassis right behind the exciter (transmitter) box. It's fairly clean, cheap and simple, w
