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

Sat Sep 20 21:25:51 EDT 2008

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" <n4lq at insightbb.com>
To: "Phil & Debbie Salas" <dpsalas at tx.rr.com>; <elecraft at 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
> 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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