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

[D C *Mac* Macdonald]

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)