How far apart do the antennas in a 2 antenna beam have to be? 108 inches? Also with the rear being only a reflector do you have to do anything special to it or just screw it in and let it sit there? Also I have heard the rear dead antenna should be higher than the driven front one,higher by how much? Will a couple inches do the trick? I am thinking about taking 2 102 whips and trying to make a mobile beam out of them for an experiment if nothing else.Hey neighbor,do you still have my other 102 laying around somewhere? I tell ya I need to stay away from the keydowns,I always come back with wild ideas Shawn

Robbed this from another forum...


"First step is to determine if the second antenna is going to be a passive element, much like that of a base station beam, or will both antennas be active? For an active system where coax will be strung to both antennas and the RF fed to both, The minimum spacing will be 1/8 wavelength. That will be about 54.5 inches at 27.105 mc. That is the easy part. The hard part is to decide if you want the main antenna fed in phase or out of phase. If out of phase, then by how much?


For Antennas spaced at 1/8 wavelength apart: ~ 54 inches in phase 0.3 dB gain omni pattern,

45 degrees 0.9 dB gain somewhat less omni

90 degrees 2.3 dB gain more pronounced lobe in the line of the antennas

135 degrees 4.2 dB gain diamond pattern, 180 degrees 3.8 dB gain bidirectional, figure 8 pattern


For antennas spaced 1/4 wavelength: ~ 108 inches,

In phase 1.1 dB gain somewhat flatten pattern perpendicular to the array.

45 degrees 1.6 dB gain still perpendicular to the array with some energy off the front.

90 degrees 3.1 dB gains again a diamond pattern in line with the array.

135 degrees 4.5 dB gain again the pattern generated looks more like a mushroom

180 degrees 3.6 dB gain bidirectional in the line of the two antennas.


There are patterns for 3/8 wavelength spacing. If you have a vehicle bigger than a compact car then those will yield similar patterns and gain. Now how do you accomplish the phasing? The easiest is to use lengths of coax that are carefully measured and cut to the desired lengths.

For 0 degrees both coaxes are the same length

45 degrees one coax will be 1/8 wavelength longer.

90 degrees one coax will be 1/4 wavelength longer.

135 degrees one coax will be 3/8 wavelength longer.

180 degrees one coax will be 1/2 wavelength longer.

Both coaxes will be 50 Ohms. It is best to make the coaxes 1/2 wavelength. The longer coax or the one that will phase shift will be longer by the lengths above.

The above method may not yield the gains or patterns listed due to phase errors in the antenna currents. This is due to the phase shifts of the voltage and the current in the coax which will vary with length and load. This method while the easiest generally yield the widest variation from the listed patterns and gains.

The best method is feed both antennas with 1/4 wavelength coax. At the junction where the main feed tees in, place a series inductor and a shunt capacitor in one of the feed lines to one of the antennas. By making the inductor or the capacitor variable or both, the phase of the current in one antenna can be adjusted to a pattern and gain that you wish. This method requires more construction techniques and some means of monitoring the phase adjustments.

It is possible to get as much as 4.5 dB gain by phasing two or more antennas on a vehicle. With the first method you get what you get. It is dependant on how well you measure the coax and the antennas themselves. Also a word of caution. Use the same type of coax for both legs of the phasing harness to each antenna. You must also account for the velocity factor of the coax. This also adds a variable that is not adjustable in the first method."

With that said, a shorter "dead" antenna will cause the radiation pattern to be drawn towards it...a taller "dead" antenna will cause the pattern to be pushed away from it...

I accidentally found this out running a 102 behind my W5000 (the only way I could get a good SWR at the time)...it SMOKED to the front, but was dead to the rear.

That was really good reading, Steved. Thanks for posting that up!

[quote="outlawmobile"].Hey neighbor,do you still have my other 102 laying around somewhere? /quote]


It's in the garage. The dead use it sometimes to transmit signals to me in the house.
You can go get it if you need it.......but you'll have to pry it from their dead hands.....

[quote="Night Hawk"]

outlawmobile wrote:

.Hey neighbor,do you still have my other 102 laying around somewhere? /quote] It's in the garage. The dead use it sometimes to transmit signals to me in the house. You can go get it if you need it.......but you'll have to pry it from their dead hands.....

Oh hell no,in that case you can keep it.I will just go to Rat Shack and get another Shawn

_________________
Snowman...The Real #1 around the Keystone.You will be missed...You will not be forgotten.R.I.P my friend.

[quote="outlawmobile"]

Night Hawk wrote:

outlawmobile wrote: .Hey neighbor,do you still have my other 102 laying around somewhere? /quote] It's in the garage. The dead use it sometimes to transmit signals to me in the house. You can go get it if you need it.......but you'll have to pry it from their dead hands..... Oh hell no,in that case you can keep it.I will just go to Rat Shack and get another Shawn

shaun what are you afraid of?

[quote="bluebird"]

outlawmobile wrote:

Night Hawk wrote: outlawmobile wrote: .Hey neighbor,do you still have my other 102 laying around somewhere? /quote] It's in the garage. The dead use it sometimes to transmit signals to me in the house. You can go get it if you need it.......but you'll have to pry it from their dead hands..... Oh hell no,in that case you can keep it.I will just go to Rat Shack and get another Shawn shaun what are you afraid of?

There are dead people in Nighthawks garage.I sh!& you not there is a cooler that the funeral home keeps bodies in in the same barn style building where his garage space is. I aint goin nowhere near his garage for no reason. Shawn

_________________
Snowman...The Real #1 around the Keystone.You will be missed...You will not be forgotten.R.I.P my friend.

Let me add my 2 cents to this.
I tried this idea.
my swr went fvckin nuts when i tried to add the parasitic directing element to the mobile.
Not saying you cant do this.
But if you to only realize 1db of gain in the forward direction, it would not be a noticible worthwhile gain.
4.5db gain might be worth the work, but on your mobile you're looking at putting an antenna on the hood and the trunk and the roof's groundplain will mess that up.
Oh and the ghosts just want your a$$, so unless you are afraid of polterghiest **** you will be ok.

<- see he is one of them.......