Re: HT Counterpoise - "Tiger Tails"
Jef - N5JEF
toggle quoted messageShow quoted text
It's nice to see someone taking the time to make measurements, but there are several serious inaccuracies in this article. Too bad the testing was not carried out as scientifically as that other recent example of antenna testing, forwarded by Jim N6MED.
"As most of us know, a transmitted signal works best with a resonant antenna specific to that frequency.
"A resonant antenna typically means the best radiation pattern off the antenna and a low SWR (among other things, I will not go into too much detail)."
Simply not true, and misleading, but a very common example of ham lore.
For an antenna to be resonant simply means that it is at a length where the inductive and capacitive reactance cancel each other. Multiples of a quarter wave will be resonant, but can have hugely different impedance (thus SWR) and radiation pattern. A quarter-wave monopole such as is common for VHF on the roof of a vehicle, when resonant, will have a feed-point impedance impedance of around 37-ohms. This, matched to a 50-ohm feedline, means you will always have some reflected power and never achieve an actual 1:1 SWR. A half-wave dipole in free space has a feedpoint impedance of about 73-ohms, so again, you never have a 1:1 SWR, although bringing the dipole closer to earth will lower it's impedance and bring it closer to 1:1 SWR, while also distorting its radiation pattern. The popular 5/8 wave mobile whip is not resonant at all, but it can sometimes provide advantages over a 1/4-wave resonant whip due to its additional length, and by getting the region of maximum antenna current higher above the vehicle. The popular double-extended Zep (a 1 1/4 wave doublet) has the most lateral gain of any doublet, but it is certainly not resonant.
Resonance and SWR have absolutely nothing directly to do with antenna radiation pattern or radiation efficiency, but are important in terms of feedline loss and matching to the radio equipment.
Antenna efficiency is simply the ratio of power applied / power lost as heat. It has nothing to do with SWR, resonance, or antenna pattern. This is why a small 1/10 wavelength loop antenna can approach the same efficiency as a full size dipole if the conducting surface is sufficient to minimize loss by heating. Power radiated is proportional to the movement of charge times the lineal distance of the movement.
Antenna pattern has to do with the geometry of the antenna and the field(s) produced. A half-wave dipole is the simplest, but the a 1 1/4 wave doublet has 3dB more gain (and needs impedance matching.) Any wire longer than about 1 1/4 wave begins to produce lobes and nulls, that increasingly direct more energy along the axis of the antenna. Longer is not necessarily better when it comes to antennas.
The height of an antenna above ground can have huge effect on the radiation pattern due to constructive or destructive interference with the ground.
And so on...
"it is not possible to have a full quarter (over 19 inches) or half wave (19 inches times two) with a handheld that is only approximately 4 inches tall itself."
It certainly is possible at VHF, but not always practical, as shown recently in the video forwarded by Jim. And a full quarter wave at UHF is the norm.
You know, I was going to take the time to comment on a few other blatant misconceptions in the article, but I feel that the amount of time to do so probably outweighs anyone's interest in the subject.
Let me just cut to the chase here and point out that he had both transmitter and receiver well within the near field (only 11 feet apart) and he did not account in any way for height or reflections. I'm sure he saw widely varying numbers--both positive and negative--but he did not even mention them, and reported only the positive.
If anyone cares, take a look at the video Jim linked to and the excellent theoretical and practical concerns that we accounted for there.
This is the kind of well-meaning crap that gives amateurs a bad reputation among professionals.
I care--a lot--because I think there is an important role for amateurs in emergency preparedness and advancing the state of the art in areas that might be overlooked by more commercial interests, but sheesh, there is far more baloney than thoughtful, informed knowledge getting put out and regurgitated.
- Jef N5JEF