[TowerTalk] Balun Recommendation
W4EF at dellroy.com
Sat Apr 14 10:42:21 PDT 2012
Guys, I was testing their Unun. Although I do agree with what K9YC says
about testing common-mode chokes, I just want to be clear that is not
what I was testing. Also, I think thirty seconds key down at 1KW is a
pretty good test for something with small thermal mass. I would have
gone longer, but my amp is a Drake L-7 and I would be leery of running
it longer than that (even 30 seconds is probably questionable). A loss
of 0.2dB would be ~50 watts dissipation in the Unun. I claim that much
dissipation would produce a noticeable temperature rise in the unun
core/windings after 30 seconds. The goal of the test was to give some
indication of whether the unun was going deliver the bulk of the input
power to the antenna. On that score it passed IMO. I also didn't see any
drift in VSWR while doing the CQ 160 CW contest, where contrary to what
W6RMK would have you all believe (he's heard to many of my "fish
stories"), I do CQ a lot with no answers :-)
73, Mike W4EF...
> On 4/14/12 9:09 AM, Jim Brown wrote:
>> On 4/14/2012 8:49 AM, Hank Garretson wrote:
>>> I gently suggest that a more rigorous test is in order.
> OK, Mike.. Hank isn't going for your calibrated fingertip..
> I know you always get a response on your first call, and nobody ever
> asks for a repeat, so your transmit duty cycle is low. (how long does it
> really take to send QRZ for the next guy in the pileup waiting to work
> you?) SO maybe your usage is unrealistic for the rest of the world?<grin>
> There are those little stick on liquid crystal thermometers. You could
> stick one on, close up the box. Put the brick on the key for a few
> minutes, then go out and check?
> THis brings up an interesting question of testing this kind of thing.
> When I used to test motors and transformers, we'd do it by measuring the
> DC resistance of the wires. But that's the temperature of the wire, and
> what you're really interested in is the temperature of the core. I
> guess if you reach thermal equilibrium, it's probably pretty close.
> The other idea is to get those temp-il-stik crayons.. They change color
> (permanently) over a fairly narrow temperature range. One could mark
> the core, make your prototype, blast away, and then see if it got too hot.
> One could also make a test article with a temperature probe built into
> it. This might be a good way for one of those "encased in PVC pipe"
> style devices.
> A lot of the claims of power handling around are basically in the nature
> of "I hung this up on a test antenna and ran it at a kW for some time
> and it didn't fail" without a lot of control for things like air
> temperature, wind, level of mismatch/choking needed, etc.
> Doing actual temperature measurements in a controlled environment, with
> controlled test conditions would be pretty novel and something that it
> seems very few manufacturers in the ham market does: providing
> meaningful power handling data in a controlled environment. All of the
> "kilowatt balun" sellers would likely start sharpening their pitchforks
> and warming up the torch fuel because you'd be raising the bar.
>> The most effective common mode choke is a low Q resonant circuit
>> operated near resonance, so it is predominantly resistive device at
>> frequencies where it is used. The most rigourous test of a common mode
>> choke is one that applies the greatest common mode voltage across it.
>> The common mode voltage will be proportional to the imbalance in the
>> system as a whole, and will be strongly dependent on the length of the
>> transmission line into which it is inserted. Off-center-fed antennas
>> can place very high common mode voltages across a choke, and are
>> notorious for destroying them. :)
> And Mike's vertical would be a fairly good example of "off center fed",
> wouldn't it?
>> The most rigourous test that I've been able to set up was to use a choke
>> as the end insulator of an end-fed dipole that used the coax shield as
>> one half of the dipole, then transmitting at 1.5kW until I started
>> seeing degradation of SWR that indicated overheating. There's a
>> description of that antenna and those tests in the Power Point on Coax
>> Chokes on my website. It also includes an NEC analysis of the
>> dissipation that can occur with an off-center-fed antenna.
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