[AMRadio] Re: new antenna and matchbox a bust.


Geoff/W5OMR ars.w5omr at gmail.com
Tue Jan 30 22:21:43 EST 2007


John Lawson wrote:
> On Tue, 30 Jan 2007, ne1s wrote:
>
>> So what's not balanced?
>   Given two lengths of coax, connected as per the Original Description 
> - center 'hot', shields connected at each end and grounded:
>
>   Resistively - perhaps at dc-to-some-very-low-frequency - this 
> configuration will handle a 'balanced' circuit, in which both 
> conductors are 'hot' and both are referenced equally to ground.
>
>   As the frequency increases, the lumped-constant parameters of the 
> device as a whole comes into play - just what kind of play, I haven't 
> a good idea (yet).  Also - this is a fairly complex transformer, IMHO.

how?  It's not as if the frequency is going to matter. (3.5 to 7..3Mc)  
The two lines run parallel and maintain an equal distance, all the way 
from the open-wire output of the tuner, to the antenna.  The only thing 
to think about, is the velocity factor of copper wire for 'x' amount of 
feet, because the shield isn't connected and won't mean squat to the 
feed line impedance.  In fact the distance between the two conductors 
(that are acting as the feed-line) stays constant, with the physical 
characteristics of the coax to give thanks for it happening. 

Brett said that there was no difference between having the two shields 
tied together or not.  This tells me that the Z of the feed line didn't 
change, or if it did change, it didn't change enough to be noticed.

>   And while I'm not sure if parallel-adjacent coaxial cables retain 
> the characteristics we call 'balanced' at HF RF freequencies - its for 
> Damn Sure that terminating this device in a balanced-output antenna 
> tuner on one end, and in a run of classic ladder-line (of some 
> signifcantly other impedance) at the antenna end - all this forms some 
> kind of complex transformer / attenuator / network - the exact 
> electrical nature of which I haven't the skills to accurately infer at 
> this moment.
>
>   I do know it's probably not "optimal".

Speculation.  You've not tried it, and the shielding doesn't come into 
play, because nothing was affected with it connected, or not.  Ok, ok... 
I've driving that point far enough ;)

>
>   I also know that if, as a classic 4-terminal black box, you like 
> what comes out as a result of what goes in... and 'not blowing stuff 
> up' is a part of "like" - then I'd be of the opinion that It Ain't Broke.
>
>   So don't fix it.....   ;}

'Zactly!  NOW yer gettin' it! ;-)


MY 250TH rig was originally built with a Real, Live Faraday shielded 
link.  The shielding link had two coaxial outputs, withi PL-259's on 
each.  They, in turn, went to a pair of Chassis Feed-thru SO-239's.  I'm 
told that the antenna was balanced line (much like what Brett is doing) 
and the match was obtained by moving the link in/out.

Tuners?  Dem ol' guys didn't -need- no steenkin' tooner!  Just give me 
the knowledge of the voltage on the plates, the current drawn from the 
plates, and an RF Current meter, and I can tell you how much power is 
being produced.

Unfortunately, after the rig was obtained, one of the outputs of the 
link was cut short, shorted from center to shield, and now the two 
outputs are the center conductor of the RG-58 sized stuff and ground.

--
73, etc...
-Geoff/W5OMR




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