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Re: [TowerTalk] Distance from exciter, amp, tuner, dummy load, antenna

To: "Tower Talk" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Distance from exciter, amp, tuner, dummy load, antenna
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Mon, 09 Aug 2010 08:59:23 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Mon, 09 Aug 2010 00:47:48 -0400, Roger (K8RI) wrote:

>Any thoughts and suggestions? 

Hi Roger,

Several thoughts. First, my amps are manual tuned (Ten Tec Titan 425), and 
I use manual tuners (Ten Tec 229, 238). I write down all the settings for 
each antenna and band so I can go to those presets when changing bands. 
That gets me close, then I tune the tuners with exciter power, usually 
about 40W, then fire up the amp and tweak it for maximum smoke as indicated 
on the SWR meter. That minimizes distortion in the amp. With practice, I 
can do this in a couple of seconds for the tuner with exciter power, 
another few seconds with the amp (because the antenna is already matched). 

Many years ago, I tried what you want to do, and what I do now works better 
and faster. 

Here's one of the problems that may be affecting your situation. If you 
look inside most antenna tuners (including mine), you'll find switching for 
antennas with single wires running to a rotary switch, depending on the 
chassis for a return. That is LOUSY PRACTICE, and it puts a big 
discontinuity in the line that may be no big deal on 160 and 80, but can 
create significant mismatches on the higher bands. Interior wiring SHOULD 
run on coax between the SO239 connectors, the switch(es), and the tuning 
components. 

Ah, you say, the chassis is a complete enclosure, and it has low 
resistance. Yes, BUT the wires to/from the switch and components have 
inductance, and that path forms a inductive loop through the circuitry to 
the chassis and back to the SO239. If you replace the wire with coax, the 
return current all flows on the wire, not on the chassis, and there's 
almost no inductance, only the coax, which is an extension of what's on the 
SO239s. The way that EMC guys look at it, LOW frequencies and DC follows 
Ohm's Law (the chassis). High frequencies follow the path of least 
inductance (the braid of the coax). In this case, LOW frequencies are audio 
and below. I continue to be amazed that folks will ignore BIG, REAL issues 
like this while fixating on the imagined difference between an N connector 
and a UHF connector that only matter above 500 MHz. 

If you look inside the Ten Tec 425 amp, you'll find coax from input 
connector to relays, and from output connector to relays. Done right. But 
if you look in Ten Tec tuners, you'll see wires. Could that be part of your 
antenna switching problem? At Dayton, I saw this same problem in PalStar 
tuners that were on display, and pointed it out to a guy who said he was 
the design engineer. He looked at me like I was crazy. 

Same problem with ICE bandpass filters, which may be part of why their 
performance isn't that good. And you can't just go re-wiring with coax, 
because that will change their tuning.  

73, Jim Brown K9YC


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