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

[Phil]

Hello Mac and Mac, et al,

Thanks all,
Today I was totally surprised when the mailman knocked and delivered the 
MFJ949E, one day after it was shipped from the bay area.   And yes, I 
can now match 75 and 15 where the other one wouldn't.

I'm aware of the large mismatch likely on the ladder-line (actually 
window line).  I have a 4:1 Balun that I made to take care of feeding 
the balanced line, but I'm aware of the very wide swings in impedance 
this antenna presents.  The Balun I made is rated at 2KW pep, 1KW 
carrier, I think it can handle a mismatched 100 W :)  While mounting the 
949E under the shelf I looked at it's Balun, it's so tiny it looks like 
it'd melt down!  I wanted to tighten all the screws in the MFJ (always a 
good idea with any MFJ product).

However, all being said, this 88' dipole is by FAR the best ham antenna 
I've been able to use at this limited space QTH.  Some things I've tried 
worked a little better on a single band, but for working all bands 
75-10, this is the only thing that worked at all well (verticals are 
basically out due to local QRN/QRM from adjacent mobile homes).   Towers 
and beams are definitely out as the only restriction put on me when I 
moved in was that nothing I put up could damage adjacent property should 
it fall down (only one of the three push-up poles here present any 
possible threat).  After connecting the tuner I worked a JA on 15M 
(hadn't heard any JA's in awhile, but I hadn't looked for them), then 
worked some 10M contesters from back east.   In the last cycle JA's 
dominated the bands here on the left coast.  The JA was only about S2 
here, but he copied me Q5 and we chatted for a little while, so the 
antenna must be working. . .

73 de Phil,  KO6BB
http://ko6bb1.multiply.com/ (OTR Blog)
http://www.qsl.net/ko6bb/  (Web Page)

RADIOS:
Grundigs: Satellit 750 (2011), S-350 (2006)&   G6 (2011).
Kenwood: TS130S Transceiver (circa 1980).
Radio Shack: DX-380 digital portable (circa 1990).
Yaesu: Two FT-8800 Mobiles, VX8R Quad-Band (circa 2010).
Zenith: Royal-7000 Transoceanic (circa 1969).

ACCESSORIES: MFJ 1040C Pre-Selector, W9GR Audio DSP.
ANTENNA: 88' Long Ladder-line fed dipole, Apex at 35 feet.

Central California


On 12/11/2011 12:59 AM, mac wrote:
> 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) *
>> * * * * * * * * * * *
>>
>