When a high VSWR exists with a toroid ferrite balun due to a high inductive
or capacitive reactance and the toroid begins to heat, the losses will rise
to quite unacceptable losses and can possibly destroy the balun itself, even
tough the tuner used appears to have reduced the VSWR on the input side of
the circuit to a very low value.
Generally a 4:1 or 1:1 transformer type balun should be preferably used for
just a resistance transformation, not where there is a highly reactive
component on the output side of the balun. Lots of people "get away" with
this situation, but it isn't a very good idea. I commonly did this for
years with very large ferrite cores and a vanilla high pass "T" network
tuner (like the many MFJ and other "T" network tuners)
For the last 15 years I have used nothing but the common Parallel type
balanced line link coupled tuner configuration and had extremely good
results with the old fashioned and sometimes very cranky to get setup right
circuit.
"choke" type baluns (the ones that traditionally use a large number of
ferrite beads on a length of coaxial cable) are much less troublesome than
the transformer type. If your "balun" setup runs cool, then you probably
have hit upon a length of feeder that is "just right" and you are "OK". If
it is running warm then you are treading on dangerous ground and things may
be getting ready to surprise you one day with a catastrophic failure,
especially when you run the legal limit!
This no matter what the VSWR meter says between the tuner and the ferrite
balun in question.
73,
Sandy W5TVW
----- Original Message -----
From: "n4lq" <[EMAIL PROTECTED]>
To: "Phil & Debbie Salas" <[EMAIL PROTECTED]>; <elecraft@mailman.qth.net>
Sent: Friday, September 19, 2008 8:54 PM
Subject: Re: [Elecraft] Ferrite transformer losses, 43-foot vertical and the
K3
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
[EMAIL PROTECTED]
----- Original Message -----
From: "Phil & Debbie Salas" <[EMAIL PROTECTED]>
To: <elecraft@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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