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Re: [TowerTalk] Vacuum variable / inductor ratings for shunt fed tower

Subject: Re: [TowerTalk] Vacuum variable / inductor ratings for shunt fed tower
From: K4SAV <>
Date: Thu, 11 Jun 2009 11:06:48 -0500
List-post: <">>
I wouldn't recommend using the SteppIR elements as a capacity hat.  The 
elements are insulated from the boom and most of them won't add a lot a 
capacity.  The driven element has a 2 to 1 balun.  It also does a choke 
function. The primary to secondary capacitance of that balun will 
conduct some current and cause some dissipation.  The balun is unlikely 
to have a high impedance on 160 meters.  It will also have a lot a 
voltage across it.  I have not heard of anyone reporting a destroyed 
balun, and I have heard of some people using the SteppIR elements for 
top loading but they were not running 1500 watts.  The amount of power 
dissipation in the balun varies with each tower configuration.  As a 
worst case estimate for the single driven element extended (no trombone) 
and on a 70 ft tower and for 1500 watts continuous power, the balun 
dissipation should be somewhere between about 50 and 150 watts.  That's 
using some guesses for balun characteristics so the numbers may be in 
error, however they are large enough to scare me.  This is not a large 
balun and it doesn't have circulating air.

Then there is the controller potential problem.  The driver chips in 
that controller can withstand about 2 amps of peak current.  Voltage 
withstanding is zero.  So put more current than that thru them and you 
will need a controller repair.  I don't know the probability of this 
occurring.  If you are using coax arms to hold the cables away from the 
tower, then those cables will have more current induced in them than if 
they were taped to the tower legs.  This has the potential to create 
current into the controller.  It also has the potential to create more 
current into the rotator position pot.

If you are going to do this, I would recommend retracting the elements 
when feeding it in shunt mode.  I would also recommend disconnecting the 
controller.  I would also recommend shielded wire for the rotator.

A 70 ft tower plus 4 element SteppIR with the 6 meter option should be 
resonant around 2.4 MHz.  A 0.1 inch diameter shunt wire attached at the 
60 ft point, held off the tower by about 3 ft should do it.  You will 
need a series cap to tune of the reactance of the shunt wire.  About 110 
pf to give a good match on 1.84 MHz.  The SWR goes up as you move up the 
band.  At 1.9 MHz (92 pf) the SWR is about 1.8.  Bandwidth is narrow, 
about 35 KHz for SWR less than 2.0, assuming about 8 ohms ground loss.  
On the low end of the band and for 1500 watts, the voltage across the 
cap is about 5100 VRMS (7200 V peak) and the current is about 6.4 amps.

For 80 meters you will probably need a different shunt, unless you want 
to build a more complicated matching network using the 160 shunt wire.  
An attachment point of about 37 ft with a 70 pf cap should produce a 
good match on 3.7 MHz.  You need about 80 to 55 pf to tune the whole 
band.  Bandwidth is about 140 kHz.  At the high end of the band and for 
1500 watts, the voltage across the cap is about 3750 VRMS (5300 Vpeak) 
and the current is about 5.1 amps.

A disclaimer: These numbers may have some error.  EZNEC does a poor job 
of analyzing shunt fed towers because of the wide variation in wire 
diameters, but maybe it will get you in the ballpark.

Jerry, K4SAV

Allen R. Brier wrote:
> What are the minimum ratings required for a vacuum variable used to shunt
> feed a tower (used at full legal limit plus some head room)?
> I would like to shunt feed my 70 foot tower with a 4 el SteppIR on top
> (parasitic elements are NOT YET grounded) but with the 30/40 meter and 6
> meter options (all to be used as a capacity hat) on 80 and 160 meters. What
> capacitive (and inductive) values am I looking at approximately? I assume
> that I will have to switch in some inductance at least on 160 meters. The
> tower is a crank-up, but I will be bonding the shield to the tower at top
> and bottom plus there is an air terminal on top, also running the full
> length of the tower.
> Recommendations??
> Allen Brier N5XZ / KL5DX
> 1515 Windloch Lane
> Richmond, TX 77406-2533
> 281-342-1882
> 713-705-4801 (Cell)


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