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

[Phil & Debbie Salas]

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