Discussion in 'Amateur Radio News' started by KB7TBT, Mar 13, 2020.
Ham Radio - 630-10 meter antenna software modeling results.
'WITH RADIALS' the antenna is shown(in your video)at low MF as almost -12 dBi. That tells you that very little of the resistance is radiative. When you build it on poor-to-fair ground, with one asymmetrically positioned ground rod, your impedance will be very different and your gain will be even worse.
-12 dBi is a poor peak gain.
The small azimuthal off-omni is caused by the asymmetric distribution of radials relative to the antenna footprint.
You seem to think that good SWR is objective of an antenna-- why not just attach a dummy load if that is the case?
-12 dB, my rubber duck is more efficient than that!
They were using the word gain in the video, but they were actually referring to less loss, still substantial.
I normally don't watch these videos, and now I know why!!!
Asymmetric grounding produces directionality. It's important that you not list this as a 'plus' for your antenna: any and all ground-dependent antenna will become directional in azimuth when the ground is not symmetrically distributed. No one told you to put a ground rod , or radials, at one leg of the helix. You definitely need radials, and stick the ground at the center, with the helix tapering in to that point.
This directionality effect is well-known by some mobilers, as placement of a short 'vertical' on the car is worst at corners or bumpers, for example. There is a reason the best , albeit ugliest, mobile arrangements with ground-dependent antennas try to use the roof center. Otherwise, you get at least one (partial) null, and you seldom can control which direction you are traveling on a road.
IF you use this antenna as an INVITATION to see AND ACKNOWLEDGE the hard-won efforts of those before you, then you will have accomplished something.
Certainly such fun efforts are fine on the Zed 'front page', IMO, but, once again, 'it ain't news'.
Most of the power is being lost to ground. The helix probably has a rad resistance of a few of ohms at 470 KC's. (yeh, I LIKE the OLD designation...).
Why would anyone feed a helically wound short vertical, which is what this antenna is, with a 9:1 unun? The ARRL handbook and Bill Orr's Radio handbook both discuss helical antennas and they are indeed low impedance devices. I would not be surprised to discover that at 474 Khz the antenna had a lower impedance than 50 ohms requiring a step up rather than a step down transformer to match nicely to a 50 ohm coax.
Good points (BTW, this is called, as in the prior 'Part 1' comments, a 'normal mode helix' at the low bands).
These days, you can walk right up to your antenna connection and measure the Smith Chart at various bands. With a hand-held VNA! That answers very clearly, in situ, why a 9:1 transformer is a bandaid on a skin rash, so to speak! IOW, a 9:1 ain't a great option, and adds some ohmic loss.
A tuner , preferably installed at the antenna and remotely controlled, is a better option.
HA! This actually brought up a pleasant memory.
There are a few applications where that effect comes in quite handy.... Fox hunts for example, at anything above 30mhz or so. Mount that quarter wave at the rear of the car and you've got a rather nifty beam antenna... point the car for the best signal and follow your nose. ;-)