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Gang,
Al principio . . . I got my paws on a QKits.com’s “250 mW amplifier” kit & built it into my box of stuff with the DC Kits’ PLL stereo transmitter. Worked out great & I could actually hear the changón music that I had coming out of the computer about a mile from the house.
Yes, I turned it off & went back to nominal legality.
So I got crazy & drew up a board to hold a buffer amp, bandpass filter & the same “final” PA circuit as the QKits board and a copy of the Ramsey LPF. Had three boards made at ExpressPCB.com. Stuffed one of the boards & figured out why the latching relays I’d bought didn’t quite work right (and fixed that).
Gang,
Al principio . . . I got my paws on a QKits.com’s “250 mW amplifier” kit & built it into my box of stuff with the DC Kits’ PLL stereo transmitter. Worked out great & I could actually hear the changón music that I had coming out of the computer about a mile from the house.
Yes, I turned it off & went back to nominal legality.
So I got crazy & drew up a board to hold a buffer amp, bandpass filter & the same “final” PA circuit as the QKits board and a copy of the Ramsey LPF. Had three boards made at ExpressPCB.com. Stuffed one of the boards & figured out why the latching relays I’d bought didn’t quite work right (and fixed that).
Tuned it up a bit wide. The BPF, if tuned for the frequency I was parked on, is spectacular in cutting out the x4 harmonic (and I can hope the x2 & x3 harmonics as well). Output goes to nearly .5W. The BPF is two parallel LCs in a box with no direct coupling. The proximity of the coils in the box performs the coupling. A-la the “loose coupler” crystal set. I can tune one LC for the high end (108 MHz) & get the other LC to “pass” the lower end frequencies (88.0 MHz). Doing this gives about .25 W out.
So I’m listening to the John Coltrane “Blue Trane” CD playin’ at some jazz site in Switzerland. The signals coming through my antiquated dirt-cheap hi-fi stereo receiver/amp. Clear, clean signal. Ride cymbal & bass lines are not distorted.
And then I punch the button to switch in the RF amp set up. (The LPF’s in the circuit all the time) Suddenly the nice “wideband” AF that I’d been getting sounds muffled. Not quite playing through cotton, but it’s a noticeable change in the way the FM receiver is getting the signal.
Part of this I check off as signal overload clamping down the limiter circuits in the RF, mixer & IF chain of the receiver. (I’m gonna have to listen to this on the car radio as I leave the estate. I’ll have the groundskeeper push the QRO/QRP buttons. He’s at least that technically qualified.)
I’ve been gettin’ RF burns for over 45 years now. This is truly my first foray into VHF transmitters. Like the length of coax between the SWR meter and the antenna makes a difference. Interesting discovery, that. The coax was a quarterwave length on 107 MHz. Disconnecting it from the antenna gave no change in SWR on the meter. The coax was a tuned stub. Hmm . . .
What a trip. To think I’m just doing this so I can listen to jams while mowing the lawn. The groundskeeper doesn’t do lawns.
73
Nils
W8IJN
Total posts : 45366Hi Nils,
I have built several VHF amps (2 meters) in the 20 to 50 watt class and also found that the coax can really screw up measurements. One I remember would give almost no gain. Turns out that I had grabbed a piece of CB grade coax to couple the exciter to the amplifier. A three foot length of this stuff gave about 10 dB loss. Got some good Belden RG58 and the amp worked like it was supposed to. VHF is a different world.
One subtle problem that comes up with VHF power amps is the power supply and/or the output amplifier can oscillate. This could be affecting your audio and if it is happening you will definitely be interfering with many stations.
My sad experience with this was with a mobile amp powered from the car battery. When keyed, there was a 5MHz. oscillation at the amplifier power terminal. I fixed it as described below.
Standard practice is to bypass the supply right at the final output transistor power source (the collector choke) with a series ferrite bead and three caps in parallel to the gound plane (10 uF, 1 uF, and .1 uF). I also resorted to a neutralization feedback network from the collector to the base. This was a small cap, inductor, and resistor in series. Don’t remember the values but the ARRL book has examples.
I found the problem when I checked the 13.8 volt VCC with a scope. The oscillation was obvious.
Hope this helps.
73
Neil
Total posts : 45366Neil,
Thanks for the hints & info. When I got into this design thing on the amp/switch/LPF &c board design, I ended up with bypass caps on the board that weren’t on my reference schemo. I went nuts with the little puppies and parked .01s and .001s all over the board on various DC lines. That and a handful of 10 & 100 uF tantalums. So I figured I was pretty good there. Also I used the ferrite bead chokes that were part of the QKits.com boards. (The kits are really from KitsRus, based on many of SM0VPO’s designs.)
Putting each section in its own little tin strip box also saved me many interaction episodes (usually resulting in me having to replace the window screen near this desk). I did not, however, put a bead on the base lead to the 2N3866 clone that comes with the Qkits board.
Last night, after I’d gibbered here, I pulled out the tuning tools & tuned the BPF to the frequency I was parked on (107.3 MHz). That was a big help in giving the signal a “no so smothered” tener, which was the game plan at that time.
I’ve also built into the box, using RG174 (yeah, I know, Ethernet hose is best), a 2*f stub filter. Putting that in dropped my apparent output in half, which is either a curse or blessing, depending on whether or not I blammo the neighbor’s VHF fire/rescue HT. (He’s one of the local fire fighters & I don’t wanna get in their way, even if only from my back yard out to the street.)
So backing up & doing a retune helped greatly. So much for the “no tune” style of radio that I’ve gotten used to since I worked on the TR7 line at Drake something like 25 years back. Hard to believe that my TR7 is that old. But then the morning paper informed me that, as I hit the 60 mark, I’m joining a list of survivors.
Sweet. Now can I stay up late to watch reruns of “Miami Vice”?
73
Nils
W8IJN
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