[1000mp] MPV Driving Amp

Rob Atkinson k5uj at sbcglobal.net
Mon Oct 23 09:13:47 EDT 2006 

mike, thanks for the explanation; very interesting.    i wonder if the 
mk v's broadband solid state pa (as opposed to the collins hi z tube pa 
and variable tank circuit) will affect this solution.  I do not know 
anything about the 30s1 input.

rob / k5uj



On Sunday, October 22, 2006, at 08:30 PM, Mike Schatzberg wrote:

> 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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