This 'problem' has the obvous solution of ' just get a bigger balun'.
Then, instead of agonizing over test methods, we can operate the radios.
Use two cores instead of one and forget about taking temperatures.
Jim Lux wrote:
>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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