[Elecraft] Ferrite transformer losses, 43-foot vertical and the K3

[n4lq]

Phil. Questions:
1. Why would one use a balun when both the antenna and coax are unbalanced? 
Wouldn't a unun be appropriate?
2. What are the swrs at the balun? The swr at the K3 doesn't tell us much 
since the length of the coax affects it greatly.
Steve Ellington N4LQ
N4LQ at insightbb.com
----- Original Message ----- 
From: "Phil & Debbie Salas" <dpsalas at tx.rr.com>
To: <elecraft at mailman.qth.net>
Sent: Friday, September 19, 2008 4:21 PM
Subject: [Elecraft] Ferrite transformer losses, 43-foot vertical and the K3


> Because of some earlier discussions here, I wanted to actually measure
> losses in a 4:1 ferrite transformer.  I wanted this info as I have a
> home-brew 43-foot vertical and these transformers are what seem to be
> recommended for "matching" to this antenna.
>
> I used a FT240-61 ferrite toroid which has a permeability of 125.  I chose
> 16-gauge speaker wire to experiment with.  This is because I want to
> eventually use high voltage wire, and 16-gauge is the largest gauge
> 2-conductor high-voltage wire I could find (McMaster-Carr 9634T701 @
> $2.65/foot).  I built a 4:1 unun, as I am feeding an unbalanced vertical
> antenna.  And I decided to go with a voltage balun as this is a simpler
> structure than a current balun or unun.
>
> With a little experimentation, I was able to build a very good 1.8-30 MHz
> 4:1 unun.  This consists of 12-turns of the 16-gauge speaker wire on the
> FT-240-61.  As the voltage balun is a little inductive causing degradation
> at the higher frequencies, I tuned this out with a 33pf capacitor across 
> the
> 50 ohm input.  This gave me a transformer with 1.2:1 SWR at 1.8 MHz, but
> less than 1.1:1 from 3.5-30 MHz.  In order to measure loss, I built a 
> second
> identical transformer and connected these back-to-back.  I measured
> insertion loss with both an Array Solutions PowerMaster, and a Tektronix
> TDS-2200 digital oscilloscope.  I made all measurements with 20 watts of 
> RF
> power on my workbench.  Bottom line:  Loss through both transformers was
> less than ½-watt (20 watts forward power) from 1.8-30 MHz.  This is just a
> little over 1% of loss in each transformer.  Even if my measurements are 
> off
> by a factor of two, this is still pretty much insignificant loss.
>
> Next I installed one of these transformers at the base of my 43-foot
> vertical.  My radial system isn't the best in the world because of the 
> space
> I'm restricted to.  I have about a dozen random-length radials with 
> lengths
> up to about 50-feet.  My transmission line is 60-feet of Andrew ½-inch
> Heliax that transitions to a 3-foot section of LMR-400 inside my house 
> going
> to the K3.  My Array Solutions PowerMaster is located immediately at the
> output of the K3.  The SWR measured with the PowerMaster was as follows:
>
> 160:  4.9:1
> 80:  6.3:1
> 60:  3.3:1
> 40:  3.2:1
> 30:  3.2:1
> 20:  3:1
> 17:  2.1:1
> 15:  1.9:1
> 12:  1.4:1
> 10:  2.2:1
>
> Obviously, these mismatches are easily handled by the internal K3
> auto-tuner.  And line loss is minimal because the mismatch isn't very 
> high,
> and the transmission line is very low loss.
>
> The 16-gauge speaker wire on the FT240-61 core seems to be working fine 
> even
> with 600 watts out of my ALS-600 amplifier.  However, I do have some of 
> that
> expensive McMaster-Carr high-voltage wire on order.
>
> Anyway, I just thought I'd share these measurements with the group.
>
> Phil - AD5X
>
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