To answer one question "And what's so magic about 5/8 wave antennas? It's
not even resonant,
so obviously a matching network of some sort will be needed, but why
5/8? Why not 11/16? Or 3/4? Or something else?"


So why would anyone use a 5/8 wave antenna if they have to go through all
that extra work? After all, a ground plane antenna provides a nicer match.
There are a couple of answers. The first is GAIN. The computer shows that
the antenna (mounted 1 foot above ground) has a gain of about 1.5 dBd higher
than a dipole's gain (also mounted 1 foot above ground.)

The second reason you may want to use the 5/8 wave vertical is to obtain a
lower angle of radiation. A half wave antenna's radiation peak angle is 20
degrees. You'll find that the 5/8 wave antenna's angle of radiation is just
16 degrees making it an even better dx antenna.

See URL: http://ac3l.com/five8th.htm

SOME MORE INFO
About Mobile Dual-Band 2m/440 -- Roger K6XQ advises - Can I build a
2m/440mhz 5/8 mobile antenna? A 1/4 wave on 144 MHz is 3/4 wave on 440. The
SWR is usually acceptable on the 3rd harmonic, although that is partly due
to the increased feeder losses on 440. But SWR is not the whole story. A 3/4
wave radiator on 440 has radiation lobes that send some of your signal at
high angles, not toward the horizon where you probably want it. This is
usually not so severe to make the antenna unusable, and may be considered a
reasonable compromise for the dual band coverage. A 5/8 wave antenna on 144
MHz is 15/8 waves on 440. If you do the simple math you will see that on 440
you will have almost a 2 wavelength antenna, which is becoming what we call
a "long wire" on the HF bands. As you probably know, a long wire radiates
with multiple lobes, the largest of which are toward the ends of the wire.
In other words, most of your signal will be radiated up toward the sky. The
reason for the coil in commercial dual band antennas is to phase the antenna
currents so you actually end up with two colinear elements which both
radiate at low angles, reinforcing each other and providing gain. Without
the phasing coil you get multiple undesirable lobes. Also keep in mind that
5/8 wave is NOT a resonant length. There are several ways to compensate for
the reactance at the base of a 5/8 wave radiator, the easiest being to add a
small inductance in series which makes the antenna electrically equivalent
to a 3/4 wave. (3/4 wave is resonant, as you have already discovered). Given
that the gain advantage of a 5/8 wave antenna over a 1/4 wave antenna is
probably not noticeable except under the most difficult conditions, and even
then may or may not make a difference, my suggestion is to stick with the
1/4 wave antenna. It is simple and works OK.


Above 5/8 wave the antenna exibits split lobes - so 5/8 is considered the
optimum

Yes it requires a matching network - but easy to implement

II use a quarter wave around my area as the repeaters are usually up on
mountain tops - thus a higher radiation works well.

On the open highways I use a 5/8 wave -- ranges way down the road.

Yep use a SWR meter and check resonance on the two bands -- now you will
have piece of mind

Wouldn't worry about how a dual band antenna is constructed - different
schemes can be used. Just so it is resonant as speced and adjusted if it can
be.


--
CL -- I doubt, therefore I might be !






"Doug McLaren" wrote in message
. ..
I've got an HT and it's got a basic dual band 144 MHz/440 MHz antenna.
It's about a meter long, which would be correct for the 144 MHz band.

What makes this antenna different from a 144 MHz (only) antenna?

Are there typically two conductors inside, with one 1 meter and one 35
cm? Or are the antennas typically just identical to a 144 MHz (only)
antenna, and it just works at 3/4 wavelength at 70 cm?

As I understand it, a 3/4 wavelength whip should work reasonably well,
being resonant and having a similar impedance to a 1/4 wavelength
whip. Is this correct?

To be more to the point -- I've got a Kenwood D700A in my car. It has
a built in duplexer, and one antenna jack. I've hooked it up to a 2
meter antenna on my roof, and it seems to work fine, both on 2m and
70cm. But I'm wondering if I'm risking ruining it ...

(I haven't put a SWR meter on it yet. I probably should.)

And what's so magic about 5/8 wave antennas? It's not even resonant,
so obviously a matching network of some sort will be needed, but why
5/8? Why not 11/16? Or 3/4? Or something else?

--
Doug McLaren, , AD5RH
`What's the use of happiness? It can't buy you money.' -Henny Youngman