[Antennas] Re: 2 meter antenna (longwire)

[Barry L. Ornitz]

Sean, WD8OKN, asked:

> Let's say I make a vertical long wire for 2 meters?  I 
> attach the braid of the coax to a 1/4 wave vertical 
> facing down and attach the center conductor to a vertical 
> long wire perhaps, let's say, 40 feet long (or high) to 
> the top of a tree.  Has anyone tried it?  It could be 
> several wave lengths long...   If I can match it to the 2 
> meter rig, and provide some decent height, would it 
> work?  What kind of gain would I realize?

Contrary to what another on the list said, longwire 
antennas work as well at VHF as they do at HF (at least 
until the point where the wire diameter is an appreciable 
fraction of the operating wavelength).  But you will not 
find the antenna you describe very useful.  Read on for 
details...

A classic longwire antenna is many wavelengths long.  In 
your example, the antenna would be a little over six 
wavelengths long so it qualifies on this account.

But the guiding principle behind longwire antennas is that 
their maximum radiation is oriented towards (but not 
exactly at) the ends.  So in the case of your antenna, most 
of its radiation would be up and down, not toward the 
horizon where you normally want it to be.

To understand the radiation pattern of a longwire antenna, 
consider the radiation pattern of an ordinary half-wave 
radiator.  This is the standard figure-8 pattern as, 
hopefully, everyone is familiar with.  Now consider a full 
wavelength radiator.  The pattern will now be cloverleaf 
shaped (4-leaf).  You can continue this on, and as you add 
length the radiation pattern adds additional lobes.  
Actually there is a lobe for every half-wavelength of wire 
length of the radiator.

If you look at the middle of the wire as the center of the 
antenna, a radiation pattern will show half of the lobes 
tilted forward and half tilted backward in the direction of 
the wire.  Since there will be a lobe for every half-
wavelength of wire, an antenna an odd number of wavelengths 
long will have an odd number of lobes.  One of these lobes 
will be perpendicular to the wire.  But since your antenna 
is approximately six wavelengths long, there will be no 
lobe pointing toward the horizon.  But even if your antenna 
was a 6.5 wavelength longwire antenna, its "gain" 
at the horizon would be less than the that of the lobes 
towards the ends of the antenna.  The largest lobes will be 
the ones closest to the directions of the wire.

You can find more details in the ARRL Antenna Book.  If you 
wish to go deeper into the theory (especially for standing 
wave versus travelling wave situations such as terminated 
long wires), I suggest LaPort's "Radio Antenna 
Engineering" which is the 'Bible' of longwire and rhombic 
design.  [You will probably have to go to an engineering 
university library for this book.  Used copies go upwards 
of several hundred dollars.  I was lucky to find my copy.]  
Another good reference would be either edition of Jasik's 
"Antenna Engineering Handbook".

>From Figure 11-1 of Jasik's 2nd edition, for a longwire of 
six wavelengths total length, the maximum lobes will occur 
20 degrees from the end of the antenna.  The second lobes 
will occur about 42 degrees from the antenna ends and the 
third lobes will be 54 degrees from the antenna ends. The 
fourth, fifth, and sixth nodes will be 65, 76, and 86 
degrees from the antenna axis respectively.

So you can see that unless you want to talk to an orbiting 
satellite, and even then for short periods of time, your 
antenna pattern is pointed in the wrong direction - up!

If you have a need for point-to-point communications, a 
longwire antenna can be quite useful on VHF without being 
very long physically.  You can get even more gain by using 
two longwire antennas in the form of a Vee with the angle 
of the Vee chosen to aid the radiation from the maximum 
lobes.  Then you can stack two of these Vee antennas to 
form a rhombic.  Without termination, all of these antennas 
are bi-directional and act as standing wave antennas.  
Proper termination changes them to travelling wave antennas 
and makes them more mono-directional.  

Unfortunately, all of these antennas require significant 
real estate and they are quite difficult to rotate!  :-)

    73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net