El lunes, 24 de junio de 2013 20:04:01 UTC+2, Dave Platt escribió:
In article ,

Jeff Liebermann wrote:



I overlooked the mobile operation from a car. Of course, when the

antenna is mounted on a metal surface, you don't need radials..



If it's a metal car roof, you don't need radials. Unfortunately, I've

had to deal with verhicles that have a fiberglass roof. Aluminum duct

tape ground plane (on the inside) to the rescue.



That will help but not entirely resolve the situation.



What I have heard, is that the theoretical gain advantage of a

5/8-wavelength monopole over a 1/4-wave monopole, is dependent on the

antenna being operated over a fairly large groundplane (one which

reaches out several wavelengths from the feedpoint). A simple set of

ground-radial "tapes" won't be big or extensive enough... and,

actually, neither will be the typical vehicle roof (at VHF wavelengths

at least).



According to these sources, in the absence of a good groundplane, the

5/8-wave monopole tends to "squint" - its highest-gain lobes are not

towards the horizon but aim upwards somewhat. Gain towards the

horizon may be *less* than a quarter-wave monopole on the same vehicle

mount.



So, the theoretical gain advantage of a 5/8-wave vehicle antenna may

not work out in practice. Testing would be required to see if there's

actually an advantage, or whether a "high gain" antenna of this sort

is actually a loss in practice because the gain is aimed in the wrong

directions.



And, I agree that for many vehicle mounting situations, a "ground

independent" antenna such as an end-fed half-wave may be the best bet.

I believe you can get these in a shortened form (with distributed or

lumped inductive loading in the center of the radiator) to keep the

height within reason... but going for a full-length end-fed radiator

would give you somewhat better gain and efficiency, if it's safe to

install on the vehicle.



--

Dave Platt AE6EO

Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior

I do _not_ wish to receive unsolicited commercial email, and I will

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To Jeff and Dave,

I agree on the end-fed half-wave. I like them but you need to take care of matching and good capacitors (high voltage breakdown).

The myth of the gain advantage of the 5/8lambda is from the AM broadcast antenna patterns where we have a large ground plane (mother earth, I am sure you both know).

I fully agree; the half-wave, and even the quarter-wave will win with real world ground planes/radials on HF/VHF/UHF terrestrial links. As long as people think "longer = better", the myth will continue and peoople keep buying 5/8 lambda verticals with radials (the pigeons like them!).

To avoid long discussion with others: I know stacking with good phasing does help to increase gain.

Wim
PA3DJS
please instruct your racing pigeon to skip abc.