[1000mp] MPV Driving Amp

Sun Oct 22 23:55:34 EDT 2006

Hello Win:

Collect some data along the way.  Let us know what amp, I assume the Mark V
will be used with the tuner off, and the SWR reading from the Mark V versus
the length of coax used.  Please also let us know the brand of RG 8X that
you are using.

One experiment is worth a thousand expert opinions.

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 11:51 PM
Subject: Re: [1000mp] MPV Driving Amp

> Mike,
>
> I will give it a try and tell you how it works out. I have some RG8X here
> that I will play with.  I will start with 23 ft and work back.
>
> Win
>
> ----- Original Message ----- 
> From: "Mike Schatzberg" <cherokeehillfarm at earthlink.net>
> To: "All about Yaesu 1000mp" <1000mp at mailman.qth.net>
> Sent: Sunday, October 22, 2006 8:30 PM
> Subject: Re: [1000mp] MPV Driving Amp
>
>
> > 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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> >>
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