On 6/27/2013 1:07 PM, Wimpie wrote:
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

boycott any company which has the gall to send me such ads!

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.


How refreshing! Thanks, Wim, for attempting to dispel myths.

John KD5YI