Do these things actually work?

I have a 2m 5/8 wave that I use mobile, and am told that a 7/8 wave will
give more gain. Can anyone confirm?

I'm told also that I can just take the 5/8 wave out and stick the 7/8 wave
in and it will load up. I doubt this personally - can anyone tell me if it
needs more or less inductance?

Any ideas where to get loading coils from in the UK?


Thanks all!

Sam
M1FJB

Samuel Hunt wrote:
Do these things actually work?

I have a 2m 5/8 wave that I use mobile, and am told that a 7/8 wave will
give more gain. Can anyone confirm?

Well, the 7/8 wave vertical has about 1-1/2 dB more gain than a 5/8
wave, if you measure that gain at the angle at which each antenna's
radiation is greatest. The problem is that while the 5/8 wave antenna
radiates most strongly toward the horizon (assuming a large ground
plane), the 7/8 wave radiates almost nothing at the horizon. Its
radiation is maximum at about 40 degrees above the horizon. Great for
contacting aircraft at a particular distance, but very poor for general
line-of-sight and repeater use. At the horizon, the gain of the 7/8 wave
is about 20 dB less than that of the 5/8 wave. This assumes a large
ground plane, and relative performance will vary depending on the
frequency and installation. But you can count on the 5/8 being much
better than the 7/8.

I'm told also that I can just take the 5/8 wave out and stick the 7/8 wave
in and it will load up. I doubt this personally - can anyone tell me if it
needs more or less inductance?

Considering the lousy radiation pattern, I can't see why anyone would
want to use a 7/8 wave vertical.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Samuel Hunt wrote:
Do these things actually work?

I have a 2m 5/8 wave that I use mobile, and am told that a 7/8 wave will
give more gain. Can anyone confirm?

Well, the 7/8 wave vertical has about 1-1/2 dB more gain than a 5/8 wave,
if you measure that gain at the angle at which each antenna's radiation is
greatest. The problem is that while the 5/8 wave antenna radiates most
strongly toward the horizon (assuming a large ground plane), the 7/8 wave
radiates almost nothing at the horizon. Its radiation is maximum at about
40 degrees above the horizon. Great for contacting aircraft at a
particular distance, but very poor for general line-of-sight and repeater
use. At the horizon, the gain of the 7/8 wave is about 20 dB less than
that of the 5/8 wave. This assumes a large ground plane, and relative
performance will vary depending on the frequency and installation. But you
can count on the 5/8 being much better than the 7/8.

I'm told also that I can just take the 5/8 wave out and stick the 7/8
wave in and it will load up. I doubt this personally - can anyone tell me
if it needs more or less inductance?

Considering the lousy radiation pattern, I can't see why anyone would want
to use a 7/8 wave vertical.

Roy Lewallen, W7EL

This line of reasoning (a longer wavelength is better) harks back to the
days of mobile CB antennas. Manufacturers would advertise their mobile whips
as being 1/2 wave, 5/8, 3/4 even 1 wavelength. None were even close to
being that long physically. The claim was that they had that much wire wound
up in them. One manufacturer placed a layer of aluminum foil over a portion
of the winding allowing them to claim 1/2 wavelength ( of wire) and still be
1/4 wavel resonant. In addition, the I squared R of the shorted coil
improved their BW.

Dale W4OP

Thanks all

It seems like it's false advertising claims when you see them with 6dB then,
so I'll save my money and stick with my 5/8 wave.

It's nice to run things like this past people before I get conned myself!

Sam

If you're talking about the gain and power handling claims for CB
antennas, they're pure fabrications.

Roy Lewallen, W7EL

Samuel Hunt wrote:
Thanks all

It seems like it's false advertising claims when you see them with 6dB then,
so I'll save my money and stick with my 5/8 wave.

It's nice to run things like this past people before I get conned myself!

Sam