Re: End Fed antennas w/ uBITX #ubitx

Jack Purdum
 

Jerry:

We did the same thing:

Inline image

but then put it in a quart can filled with mineral oil. It will take 250W for 5 minutes without damage to the components. (However, you could probably cook French Fries in the oil.) We added a simple display to give an idea of the power going into it:

Inline image

Jack, W8TEE

On Friday, August 3, 2018, 10:24:01 AM EDT, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:


Warren,

My dummy load is twenty 1k metal film resistors of 3 Watts.  So 50 ohms at 60 Watts.
With around 50 volts peak going in (100 volts peak-to-peak), that's on the order of  50*0.707 * (50*0.707) / 50 = 25 Watts of RF.
At times I cranked the RF up to 100 Watts.  The toroids are admittedly more massive than the resistors, but no discernible heating
in the toroids does give a little added confidence to the conclusion that we are not burning much power there.

> The cyclic VSWR pattern is very likely the result of the 100 feet of coax

As I've said earlier, when I measure my antenna system with just a few feet of coax between the matchbox and the vector impedance analyzer,
I get the same peaks and valleys in the VSWR as shown on the website but the peaks are more pronounced.   
The peaks and valleys are due to resonance in the wire, not something about the coax.  
Adding coax primarily affects frequencies at which the matchbox has a high VSWR, when losses in the coax become significant.
On a positive note, the coax does lower the VSWR at somewhat off resonant frequencies, allowing use there without a tuner.
Most non-WARC bands are usable across the entire band without a tuner even with a short coax, 80m and 10m are the primary exceptions

> It is a puzzle to me that the antenna shows resonant characteristics on non-harmonically related bands like 30 and 15 meters.

30m is near the third harmonic of 80m:  3.6*3 = 10.8mhz       Though far enough away that you still want to use a tuner with this antenna
21mhz is near the 6'th harmonic of 80m:   3.6*6 = 21.6mhz    That one's usable without a tuner.

> Additionally, you were using the "2K" model 

These are both rated at "1kW ICAS".   Danny (and his competition) does offer 2kW transformers, I have no idea what's inside them.
I have nothing here capable of more than 100W, reports are that at 1kW the toroids do heat up badly if you hold the key down too long.
Since the box is sealed and up on the roof somewhere, your primary indication is that the SWR starts rising.
On bands such as 30m where the wire is not quite resonant, you definitely don't want to go more than a few hundred Watts.


>  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.

At any integer multiple of the fundamental, the wire is resonant and purely resistive.
I'd guess that this is closer to 3000 ohms than 2500 ohms.
At resonance the measured SWR of the antenna system when using just a few feet of coax
is down at 1.2 or less, suggesting negligible losses.  (See the published charts).
Losses do rise when somewhat off resonance, as is the case with 30m.

Jerry, KE7ER


On Fri, Aug 3, 2018 at 04:46 AM, Warren Allgyer wrote:

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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