[1000mp] MPV Driving Amp

Jan Erik Holm sm2ekm at telia.com
Mon Oct 23 09:36:58 EDT 2006 

The 30S-1 has a normal PI-network input, should be 50 ohms
at the input connector if it is adjusted correctly.

/ SM2EKM
-------------------------------------------------------------------
Rob Atkinson wrote:
> 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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