|Hi John and crew, |
> John writes: I have been butchering a Heath SB220 to get it on 6. Input
VSWR is
> ~2:1 and can't seem to improve that. Rich mentions a problem with the
relay
> causing reactance.
| Yes, it's terrible at 50 MHz... I'll give out another trade secret...
Ebay constantly has rf switching and vacuum relays at auction. The rf
relays are often NOS surplus, have braided teflon type (some RG174/u and
RG400/u size) flying leads. Another great source of quality low power
<150 watts input side tr relays are surplus two-way radios like Motorola
and GE units. The last NOS pair I purchased were less than $20 US. You
also need to trash the original coax and the built in sample unit if
it's still used in the new circuit. I use 1/4 inch hard line, mini 141
hard line, Teflon type coax and/or RG 214 coax where applicable. Some
people trade out the SO-239's for N-chassis mount, I don't feel that's a
big deal, but you will need to replace them. The originals are phenolic
and not-so-hot in some applications. Your in VHF country now... |
> The bypass measurement is the same so I will get out the board burner
and add >the cap he speaks of. |
If this cap is used to cancel the "path" inductive reactance, you should
consider upgrading the components as mentioned above first for a better
circuit. The line sampler is a very rude and reactive 50 ohm
approximation at 50MHz. |
>The tank circuit is where I am perplexed! Tune cap is adjusted for
minimum
>to get max power out. I am seeing ~ 500 watts with 100 watts drive. I
>have tried removing plates from the tune cap and get identical results.
|
The tune cap should be very minimal (hopefully <8pf) at minimum. Can you
describe the rest of the output circuit values. |
>This seems to indicate that I still have too much cap in the tune side
of the
>output network. Right / Wrong? |
Coil and other tank component values..? |
>Suggestions to get more smoke out of these bottles? I thought I should
>see about 8-900 out..... |
No...... but lower drive should produce something similar to your 500
watts output. What's an amplifier if it can't make at least +10dB or
better. |
cheers
skipp
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