[1000mp] MPV Driving Amp

[Rob Atkinson]

I don't understand the logic of this method.  why a 20' length of 
jumper?    it seems to me that if you are successful with this method, 
your success will only be on one band.   on the higher bands, 20 feet 
is 1/4 or more lambda.  it seems to me that if ur amp has a tuned 
input, it makes more sense to use as short a length of amp to exciter 
jumper as possible, 3 feet let's say, and tune each band input to 1:1.  
   3' is an insignificant length even on 10 meters and makes the amp 
almost like a built-in pa with the exciter.   rearrange ur shack 
putting the rig next to the amp if you have to.   however, in my 
experience with the mk V it's a non-issue because even with a 1.5:1 
vswr into the amp, the rig's 200 w. pa can put out more than enough 
drive power for the amp even with foldback.  i've never used the tuner 
and never had a prob.   this is one of the things i like about the mk 
V.  that 200 w. pa is great with gg triode amps that need 100 w. + for 
full output.    i must say however that i do not operate 100% duty 
cycle modes.   for them a better match to the amp might be desirable.

If an amp seems to have a 2:1 swr input, I would put the amp on a dummy 
load, key it to bring the input circuit into the line and sweep the 
input with a swr analyzer to see what the swr curve is like over a 
band, so you know what the rig is really seeing when txing.  use a real 
short jumper between the amp and the analyzer to remove any variable 
induced by a longer feed.  if you are still seeing 2:1 on a band, the 
amp's input is faulty and needs to be fixed up.   My experience has 
been that trying different jumper lengths is chasing a moving target.

rob / k5uj



On Sunday, October 22, 2006, at 04:17 PM, Mike Schatzberg wrote:

> Hello Win:
>
> Since you have a good match in the bypass position and don't require 
> the
> tuner, do try the Collins approach to lower the SWR when driving the
> amplifier.
>
> Take a 20 foot length of high quality RG 8 type cable, and permanently
> install a PL 259 on one end.  Prepare the other end much the same way, 
> but
> do not install a connector.  Place the center conductor on that end
> carefully into the SO 239 input connector (or what ever input 
> connector is
> used on your amp), being sure that no shielding is shorted to the 
> center
> conductor, and it is not shorted to ground.  Use a wooden clothespin 
> to hold
> the braided shield onto the ground side of the input connector.
>
> Use low input power and tune your amplifier for maximum power output.  
> Check
> your SWR on the radio internal SWR meter.  If the match is close to 
> 1:1,
> solder another connector onto the coax and you are done.  If it isn't 
> quite
> close enough, trim about 6 inches of coax, and measure again.  You 
> should be
> able to minimize the SWR in this manner.  Then, if you have a tuned 
> input on
> your amp, further adjust the input for minimum SWR on your internal 
> meter.
> When you have finished these low power adjustments, double check your 
> work
> at full power output.
>
> 73 and Happy DXing,
>
> Mike
> W2AJI
> ----- Original Message -----
> From: "Win" <w0lz at winlyn.us>
> To: <1000mp at mailman.qth.net>
> Sent: Sunday, October 22, 2006 4:43 PM
> Subject: Re: [1000mp] MPV Driving Amp
>
>
> I think the reason I didn't have the drive I wanted was because the 
> radio
> was folding back power, because of the 2:1 SWR.  With the tuner out, I 
> have
> a 1:1 SWR through the bypass to the antenna on 15 meters.
>
> Thank you all.  I now know a lot more than I did.
>
> Win
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