[Boatanchors] (Slightly OT) A question concerning theory behind a certain antenna matcher (Kenwood AT120)

[mac]

The AT-120 is also designed to feed an unbalanced load like a coax  
line, not a balanced line like your (450 ohm?) ribbon.  No way to tell  
just what kind of  load your antenna system is presenting to the tuner  
with the information provided but apparently way beyond it's  
capabilities on 15M and also out of range on 75.

"Tuners" like the AT-120 and the AT-200 are most useful for cleaning  
up a high SWR on a coax feeder (due to antenna mismatch and/or other  
reasons) to something close to a 1:1 that Kenwood and other rice boxes  
require.

Dennis D.  W7QHO
Glendale, CA

***************

On Dec 10, 2011, at 2:45 PM, D C *Mac* Macdonald wrote:

>
> Yes, it is basically a Pi Network matching circuit. The values
> for the input capacitor and the taps on the coil are chosen to
> be correct for the 50 Ohm load to be presented to the trans-
> mitter or transceiver. The manual gives starting points for
> the "R" and "X" tune and you simply adjust those two alter-
> nately for the best match. It seems to me to be a very handy,
> if not particularly wide-ranging, matching device. It's rated
> at 20 - 300 Ohms.
>
> Making the input C and L fixed for each band should pretty well
> reduce the possibility that a "false" match could be made.
>
>
> * * * * * * * * * * *
> * 73 - Mac, K2GKK/5 *
> * k2gkk at hotmail.com *
> * (Since 30 Nov 53) *
> * Oklahoma City, OK *
> * USAF, Ret (61-81) *
> * * * * * * * * * * *
>
>
>
>> Date: Sat, 10 Dec 2011 05:30:47 +0000
>> From: ko6bb1 at gmail.com
>> To: boatanchors at mailman.qth.net
>> Subject: [Boatanchors] (Slightly OT) A question concerning theory  
>> behind
>> a certain antenna matcher (Kenwood AT120)
>>
>> Hi All,
>> I'm asking the question here as I don't belong to any antenna (or
>> Kenwood) mail groups. But the question could just as easily apply  
>> to a
>> similar tuner used with a BA :)
>>
>> Been around a long time, used various "L", "T" and even Johnson
>> Matchboxes over the years. Right now I have a Kenwood TS130S on  
>> more or
>> less permanent 'loan', along with a Kenwood AT-120 mobile tuner.  
>> Early
>> next week an MFJ949E should arrive to replace it to 'match' the 88'
>> ladder-line fed dipole to the rig.
>>
>> According to the schematic, this critter is quite different from most
>> I've used (and doesn't have nearly as wide a tuning range). It  
>> seems to
>> be centered around a classic "Pi Network" matching section, with a
>> single coil tap and fixed INPUT capacitors selected per band (75, 40,
>> 20, 15 & 10M). There is an adjustable capacitor from the output  
>> side of
>> the coil to chassis ground (these components comprise the Pi  
>> Network).
>> In addition there is a variable SERIES capacitor from the output  
>> section
>> of the matching circuit to the actual output connector. The two  
>> variable
>> capacitors in the output section are labeled "R Tune" and "X Tune".
>>
>> Now the question. Why would somebody design the tuner this way? Is it
>> because it was primarily designed to make for easy tune-up while  
>> mobile,
>> ASSUMING that the antenna used on the mobile would already be close  
>> to
>> being resonant (in other words, not need a very wide tuning range).
>> That thought would be reinforced considering that there is one
>> band-switch position per band and just two capacitors to adjust (no
>> multiple coil taps etc to fight). It matches the dipole VERY EASILY  
>> on
>> 40, 20 & 10M, but only reaches 2.5:1 on 75 and not at all on 15M,  
>> where
>> other tuners have been able to match the antenna on all bands, though
>> I'm sure with some losses.
>>
>> Or is there another reason to choose such a circuit (such as harmonic
>> reduction)?
>>
>> -- 
>> 73 de Phil, KO6BB
>> http://ko6bb1.multiply.com/ (OTR Blog)
>> http://www.qsl.net/ko6bb/ (Web Page)
> 		 	   		
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