[Antennas] Re: 2 meter antenna (longwire)

Tue, 23 Jul 2002 15:03:07 +0200

...actually I remind you that THE antenna for low angle of takeoff is the
classical 0.625 wavelentgh (5/8) antenna. It wouldn't be hard to add radials
to the vertical and hang it high up along the tree, with the height's
benefit.
		Alex   4Z5KS

-----Original Message-----
From: Dave Shrader [mailto:[email protected]] 
Sent: Tuesday, July 23, 2002 1:49 PM
To: Barry L. Ornitz
Cc: [email protected]
Subject: Re: [Antennas] Re: 2 meter antenna (longwire)


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]
> 
> - - -
> 
> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected] 
> _______________________________________________
> Antennas mailing list
> [email protected] 
> http://mailman.qth.net/mailman/listinfo/antennas
- - - 

Your moderator for this list is:
Larry Wilson KE1HZ [email protected]
_______________________________________________
Antennas mailing list
[email protected] http://mailman.qth.net/mailman/listinfo/antennas

---
Incoming mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.377 / Virus Database: 211 - Release Date: 15/07/02
 

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.377 / Virus Database: 211 - Release Date: 15/07/02