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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