"Owen Duffy" wrote in message
...
"Spin" wrote in news:tanvk.364$1a2.205@trnddc04:

I would like to make/experiment with a longwire antenna
specifically for the two meter fm band. Is there an optimum length to
try & still keep an omnidirectional pattern? Also the optimum amount
of radials and will i need a matching unit?




I am guessing that you want vertical polarisation, and omni directional
pattern in the H plane (azimuth).

As an end fed unloaded vertical wire over perfect ground is lengthened
than about 60% of a wavelength, the low angle major lobe splits and
significant power is radiated in higher angle lobes. You probably don't
want that.

So, in the sense that a longwire is an endfire antenna, it is not suited
to your need for a vertically polarised omni antenna for FM operations.

A 5/8 wave ground plane isn't a bad antenna, and about as long as is
useful in a straight radiator. It is an example of a good antenna that is
not resonant, debunking the myth that resonant antennas always work best.
It does not have a 50 ohm feedpoint, and is reactive, so some impedance
matching is needed for 50 ohm line, but it is not difficult.

For higher gain with omni pattern, you should consider a colinear array.
They are very popular, but mostly of commercial manufacture.

Owen

If you increase a single monoples length beyond 1/4 wave, the takeoff angle
of radiation compresses closer to the horizon for a gain improvement. Of
course now it isn't resonant but so what, all you have to do is finish with
a loading coil to make it look like a 3/4 w antenna and it has a resonant
low impedance again. If you increase the length beyond 5/8w though, high
angle lobes begin to appear and the low angle lobes become less intense.

Another way is to stack elements and feed them in phase so that their
patterns reinforce. There is a point of diminishing returns though due to
losses in the matching and feed systems. Commercial base antennas up to
about 10db gain are commonly available. A favorite UHF mobile antenna has
2- 5/8 sections stacked one over the other. Because the 2m and 440 FM
sections of the Ham bands are very close to harmonically related, this
antenna can be constructed to double as a 2m antenna just short of 5/8 for
that band too.

I bought a commercial #9007 KD4BUM vertical for $20 that is 5/8 over 1/4
wave collinear at about 9' long. Works great as the mount is
interchangeable with HF hamsticks and CB whips and has quite a bit of gain.
It is nothing but a fiberglass pole with #16 or 18 wire that goes up 1/4w
then windings for a phase reversal + the matching section for the next 5/8
section to look like 3/4w. Very simple in construction for such excellent
performance and still short enough for mobile use.