Home-built 5/8 wave groundplane antenna for UHF?

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Erik VanRenselaar:
I'd like to build a base antenna for use on the 440 MHz ham band and 462/467 MHz GMRS.  Is a 5/8 wave groundplane antenna a good design?  Better than a J-pole?  How about a 1/2 wave vertical dipole for UHF?

Ryan Vande Water:
1/4 wave groundplane antennas are a great mix of: performance, ease of construction, durability and cost.  for UHF, a 1/4 wave antenna wouldn't take up much space, and you could use a couple of wire coat hangers for the elements. with careful cutting and soldering, you could make an antenna that performs pretty well for under 5 bucks.

5/8 wave antennas are usually said to be "ground independent" which means (supposedly) that they work just as well without a groundplane.  I bet in practice they will work better with a groundplane.

I've been really happy with my 1/2 wave antenna on 2 meters. It's probably supposed to be mounted with a groundplane, but I've got it off the bumper of my jeep on a homemade mount.  I've worked a station about 60 miles away with 60w under normal band conditions.  It's no base station, but it seems to get out better than the 5/8 wave I had on my Altima.

Since you want to use your antenna up in the GMRS band, you'll probably want to make the center frequency of your antenna near the top of the Ham band.  Of course, there are always trade-offs when using an antenna for multiple bands.

just my 2 cents, your mileage may vary, this is my experience.  (post your disclaimer here)

Have fun building your antenna.  

KB9YNB

David Bartholomew:
I'm a bit confused... 1/4 wave and 5/8 are supposed to use a ground plane. A J-pole doesn't need one, and thus is good for base station use or on the bumper of a non metallic vehicle. The J-pole is the size of a 1/2 wave antenna, and the 5/8 wave is of course a little longer and needs a matching coil. The 5/8 gives a bit more gain and lower elevation pattern than the 1/4 wave. The 1/4 wave is shorter and easier to construct.

Dale Hunt:
Vertical antennas can get confusing, because much of the
"general knowledge" about them dates from AM broadcast usage,
where the antenna is mounted directly over an infinite ground
plane.  The same relationships don't necessarily apply to a VHF
antenna mounted several wavelengths above the ground.

A J-pole should give the same pattern and gain as a vertical
dipole.  The only difference is that the J-pole is fed at one end, but
the radiators are essentially the same.  (This presumes, of course,
that the J-pole is designed for the band in question - using a 2m
j-pole on 440 is a different matter.)  Sometimes you hear reference
to a "half wave vertical" - again, we are talking about the same
1/2 wave radiating structure, and it shouldn't make any difference
whether it is fed at one end or in the middle, as long as it is done
well and the feedline doesn't upset things too much.

Now, a quarter wave ground plane is basically a dipole with the
bottom half spread out a bit.  So it should have about the same
gain, too.  Besides being quite easy to construct (though I'd
suggest copper wire instead of steel coat hangers), spreading the
radials makes it easier to feed while keeping the feedline out of the
way.

All of these antennas so far, if installed with their tips at the same
height, should give you essentially the same performance.  The
choice among them would be based more on mechanical
considerations, etc.  In each case, the radiator is basically a half
wavelength, with maximum current 1/4 wavelength below the tip.

Now, let's look at the (in)famous 5/8 wavelength vertical.  This is a
little bit longer than a half wave, and the extra 1/8 wavelength
has relatively little current flowing in it.  (What current does flow is
out of phase with the current in the rest of the antenna.)  This says
that a 5/8 radiator, by itself, can't be that much different than a
half wave dipole (and, in fact would be a bit worse, due to the
phase cancellation.)

So why does the 5/8 wave vertical have a reputation?  Well, when
a vertical is installed over an infinite ground plane, the RF
reflecting off of this plane is in phase with the radiated signal at low
angles, thereby increasing the gain.  Raising the antenna above
the plane will increase this gain even more - up to a point.  Since
most AM broadcast towers are fed at ground level, 5/8 wave is the
height (for a single continuous conductor with the feedpoint at
ground level) that gives the highest gain.  (This is the point where
the gain achieved by increasing the height above the perfect
reflecting plane is countered by the decrease in gain due to the
current in the lower section being out of phase with that in the top
section.)

The first problem we encounter in VHF use is that we don't have
an infinite ground plane.  Even the roof of a car (perhaps one
wavelength radius) is not enough.  Let's take an example:  say I
have a 5/8 wave antenna for 2m that is 48" long.  The maximum
radiation is 1/4 down from the tip - we'll say 30" above the car
roof.  Now, at what point on the car roof would the signal need to
reflect at 30 degrees above the horizon?  30" / ( tan 30 degrees)
= 60 ", or 5' from the antenna.  This is larger than many car roofs,
and 30 degrees is much higher of an angle than is commonly used
for repeater work (unless you live near some VERY tall mountians.)

What happens if we add a set of ground plane radials instead of
mounting the 5/8 wave antenna on a flat plane?  Remember the
current at the feedpoint of the antenna is out of phase with that in
the main radiating section.  Adding a set of quarter wave radials
at the feedpoint and angling them down 45 degrees will cause the
current in the radials to be out of phase with the main part of the
antenna, too.  Now your maximum radiation is pointing upwards at
about 45 degrees, and you may even have a null at the horizon.
This might be good for working satellites, but not for terrestial work.

If you want to use a 5/8 vertical,  you need to make the radials
(or the "other half of the dipole", however it is formed) also be
5/8 wavelengths long.  Then the current in most of the top will be in
phase with the current in most of the bottom - though the center
will still be out of phase.  This is the "extended double zepp".
(See the ARRL Antenna Compendium, Vol. 1, for more info on this.)

An alternate (and better) means of adding gain is to put phase
reversing stubs in a vertical element, generally every half wave
or so.  Using this approach, you can lengthen an antenna and still
keep the maximum radiation on the horizon.


My suggestion is to start with a simple antenna, such as a quarter
wave ground plane, J-pole, or vertical dipole.  If you need more
gain, then consider going to a longer antenna, but don't bother with
a 5/8 wave radiator over 1/4 wave radials - I think you will be
quite disappointed.

When you do look for a higher gain antenna, keep these rules in
mind:  for 2m, to get 3dB omnidirectional gain over a dipole, you will
need an antenna at least 8' long.  To get 6 dB omnidirectional gain
over a dipole, you will need an antenna about 20' long.  (Of course
the corresponding lengths for 450 MHz will be about 1/3 this size.)
Any manufacturer who claims more gain for a shorter antenna is
taking liberties with the laws of physics.
manufacturer who tries to claim more gain in a shorte

Erik VanRenselaar:
Ok.  That pretty much answers my questions. The J-pole I built for 2m works well and has an SWR of about <1.2:1.  Trying it on 440 MHz results in an SWR of about 2.4:1.  I built a 445 Mhz 1/4 wave groundplane from wire and a chassis mount SO-239.  Once tuned, the SWR is about 1.3:1.  I was hoping to build a moderate gain broad-banded antenna for UHF.  For now, I think I'll buy a Diamond X-50 dual-band antenna (w/ N connector) and use that for awhile.

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