[1000mp] MPV Driving Amp

Sun Oct 22 21:30:11 EDT 2006

Yes, it is true that you may run into a compromise situation where the ideal
apparent SWR is not attained on all bands, but the troublesome band with the
highest apparent SWR is lowered significantly and the foldback of power is
eliminated.

The longer length of coax also acts to "stub" RF flowing on the shield back
to the transmitter.  This shield current alone may be confusing the SWR
metering and foldback circuit.

Collins has actually described this simple device as a kind of transformer,
which corrects the phase shift lag issues introduced by an amplifier's Pi or
T tuned inputs. You can move your transmitter as close as you want to your
amplifier, you will not overcome this issue.  Collins supplied an RG 58 type
cable, exactly 20.5 feet in length, with a warning not to shorten the cable.
This cable was used to interconnect the 30S 1 amplifier with the 32S1 or KWM
2.  If the amp was driven by the KWM 1, they supplied an additional 2.5 foot
extension, for a total length of 23 feet.  The exact length to be used is
probably most dependent upon the exact characteristic impedance of the cable
at the operating frequency and the matching networks.  Thus, for RG 8 types,
the length necessary maybe slightly different for cables of varying quality
and velocity factors and the equipment involved. These cautionary notes were
contained on page 5 of their earliest 1964 manual.

I'm sorry to have to disagree with the evaluation of the degree of
difficulty this method would produce in practice.  It is quite easy to make
such a small length of coax and to quickly experiment with different lengths
by pruning to optimize the input. Of course, you could just buy a high
quality RG 58 cable, and go with the Collins 20.5 feet and check the
results.  But as you can see from the Collins notes, not every
transmitter/amplifier combination will require the same length of cable.

This method has been used by amateurs for many, many years since made
popular by Collins.

73 and Happy DXing,

Mike
W2AJI
.

----- Original Message ----- 
From: "Rob Atkinson" <k5uj at sbcglobal.net>
To: "All about Yaesu 1000mp" <1000mp at mailman.qth.net>
Sent: Sunday, October 22, 2006 7:14 PM
Subject: Re: [1000mp] MPV Driving Amp

> 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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