Re: End Fed antennas w/ uBITX #ubitx
Gordon Gibby <ggibby@...>
Jerry --- likely that a simulator would tell you how much and how quickly reactive components add in as you move away from the exact resonant frequency. It does seem that significant reactive components might increase losses, huh? (I'm not an expert on
Any information on how far from resonant you can be before reactance increases signficnat in an end-fed, high impedance antenna, and then next, how that changes toroid losses???? that might be the $64 question.
On my homebrew off-center-fed balun diplose they "appear" more broadbanded......suggesting that the closer you get to the end, the less the importance of small variations from exact resonant frequency?
From: BITX20@groups.io <BITX20@groups.io> on behalf of Jerry Gaffke via Groups.Io <jgaffke@...>
Sent: Friday, August 3, 2018 10:23 AM
Subject: Re: [BITX20] End Fed antennas w/ uBITX
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.
On Fri, Aug 3, 2018 at 04:46 AM, Warren Allgyer wrote: