[AMRadio] antenna tuners transmision lines and more

John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Mon Apr 24 15:56:18 EDT 2006

Boy! You have to be real specific around here, HI

I should have been more specific.  I meant toroid wide band transformers not
resonated and covering the entire HF spectrum with no tuning.  I have seen
toriods used in HF tube type equipment also but they still have tuning.  I
was speaking of rigs with no internal tuning for the final amps.  Most
modern solid state equipment is this way.  This type of equipment that has
no output tuning must have a specific non reactive load attached or it will
not work as specified by the manufacture.  This is where external tuning
equipment is necessary because it is very difficult to get an antenna to be
non reactive and represent a 50 ohm load. And should you achieve this then
it would only be for a small range of frequencies.  Where as, if you had a
rig with adjustable output circuitry such as a Pi-Net with a loading and a
plate tune knob then you would be able to match a much larger range of
frequencies even though the VSWR on the coax line may be as high as 2:1.  As
Don, and perhaps another, have pointed out, these tube type rigs with build
in load and tune controls are really just built in antenna tuners.  As with
any antenna tuner they have a limited range of Z and reactance that they can
compensate for.  It may be necessary to have another tuner external for
matching balanced line etc.  And as with many things there is specific type
of circuits for external tuners that will do better jobs than others for a
specific task.  There are tuners specifically made that will match a balance
line of 600 ohms to 6000 ohms to a 50 ohm source they may have a limit with
in that as to the amount of reactance they can compensate for.  Other tuners
might be better at matching lower impedance loads.  The circuitry needs to
change for the best results.   

Now, there may be something that I have not seen that has tubes for output
and yet no tuning on the final and that requires a specific load resistance
for Z match as does the SS equipment.


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Mike Dorworth, K4XM
Sent: Monday, April 24, 2006 11:37 AM
To: Discussion of AM Radio
Subject: Re: [AMRadio] antenna tuners transmision lines and more

> Actually there are a number of commercially manufactured tube
> RF finals that DO indded use toroidal transformers.
> Dentron, for one example - they made a number of linear amps
> like that, and they were/are not alone.
The Alpha 374 had nothing but toroids
. It might be remembered as the legal
limit amp that ran for months with a brick on the key.. I bet most all
Alphas are the same..Had 3 each 8874's..

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