[Antennas] Re: 2 meter antenna (longwire)

2002年7月23日 07:48:53 -0400

Absolutely agree !!!
Deacon Dave, W1MCE
Manager, Textron Antenna Range, retired
"Barry L. Ornitz" wrote:
>> 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 [email protected]
>> - - -
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