Re: End Fed antennas w/ uBITX #ubitx

Warren Allgyer <allgyer@...>
 

Jerry and Gordon

Great data Jerry! I too am scratching my head over some the entries but it is a start and good to know.

I don't understand the impedance variation of the terminated pair and the terminated transformers individually. My test did not show that and it raises a bit of a red flag. I also looked more closely at the SWR table published by the manufacturer. Having stared at similar VIA outputs in my lab I noted two things. The cyclic VSWR pattern is very likely the result of the 100 feet of coax, very close to a half wave at 80 meters, alternating between a quarter wave transformer and a half wave at harmonics of 80 meters or whatever is the actual length of the coax. Notice that these peaks are attenuated as you approach 30 MHz. That is indicative of the loss in the system at those frequencies. The lower the loss of the transformer the higher and more uniform would be these peaks.

A final note on the heat: the numbers you show indicate losses on each individual transformer between 0.2 and 0.8 dB which is significantly better than I measured using the spectrum analyzer/tracking generator. As such I would not expect much heating from the 5-8 watts of input power you used. Additionally, you were using the "2K" model which uses a double stacked transformer for additional heat dissipation capability. And this combo was terminated in a perfect resistive load. When terminated in the wire the load will be very different and highly reactive on some bands. As such the transformer losses will rise dramatically but the apparent VSWR will not due to the losses. I sure would like to see a VIA plot of the wire itself through the transformer.

Your antenna appears to accept power much better than I suspected. The strategically positioned 6 turn air wound coil has a lot to do with that because it is what allows the wire length to change electrically and let harmonically related bands fall into the integer related even harmonic. It is a puzzle to me that the antenna shows resonant characteristics on non-harmonically related bands like 30 and 15 meters. That may be the witchcraft element of the coil combined with a strategic length of coax feedline.

Thank you for doing and reporting on all of that work! Great job.

WA8TOD

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