Antenna Spacing

[flintstone mop]

Hello folks,
In my spacious backyard I presently have a Cushcraft MA80/40 vertical. It's about 110 feet from the house. I am thinking of erecting a second vertical for 160M. It will be the MA160. Yup, I'm keeping Hamdom alive on my meager retirement salary. What's $200??? These are top loaded verticals and I have had alot of DX fun with legal limit SSB in the DX Window on 75 Meters and would like to diddle with the DX window or CW on 160M. ( I want to pretend to be Don, K4KYV, and we can compare signals)  A Hi-Tower would be twice the cost of both of the Cushcraft aerials. The Hi-Tower claim-to-fame was a hot antenna for 80 Meters. A coil is added at the bottom for your usual bottom fed vertical.
 
Question NUMBER 1: How far should the two antennas be from each other to minimize interaction?
 
Question NUMBER 2: This will be a VERY long run of coax to the house. Possibly 200 feet or more. I'm guessing the loss shouldn't hurt me that bad. OR WILL IT?
I got the space, let me play radio. HA!

Thanks for any thoughts out here.

Fred

[K1JJ]

Hi Fred,


Normally the 160M vertical won't see the 75M vertical, but the 75M will probably see the 160M and create a small "bump" f-b in the pattern.   

Also, unless you are using a full blown vertical system like Don/KYV, a simple 1/2 wave dipole will do better within 1000 miles or so. If the dipole and vertical are within 250', the coupling may be enough that they are really the same antenna, thus no difference between them.

Here's some details:

The 160M antenna can be placed close to the 75M IF you make sure it is not coupling power from the 75M vertical when not in use and vice versa.

One way is to put a sensitive SWR meter in series with the base to ground and see how much energy is coming in when you put 100W into the other. (do this test even though supposedly they are not 1/2 wave resonant to one another - ya never know what is happening out there)

There are ways to detune an antenna when not in use. 

For example, many guys use their towers as support to hold out four wire verticals in a four square for 75 or 160M. The tower is ALMOST electrically invisible once they run a wire down from the tapped top and use a coil or cap to tune the tower against ground - to move it away from the intended operating frequency. There are many ways to do it.

In your case, it may be as simple as putting a wire short or cap or coil across the PL-259 connector of the unused vertical. This will act like a stub to tune it somewhere away from the freq used. Using the SWR meter in line while dumping power into the other antenna, and adjusting this "stub"  will show you the null you are looking for.  Do the same for the other ant. Then you just add the "stub" when the ant is not in use.

However, anytime you add a large piece of wire, etc, behind an antenna, even if it is not resonant, you will get a slight bump/null in the pattern, even if it is a small  db or so. This is the lazy pattern effect f-b created as a reflector or even as a director on the opposite side.  The element does not have to be tuned right on as a ref or dir.  Just pull it (resonate it in freq) as far away as you can for min power coupling as described above and you'll do the best you can for the situation.

As for the 200' coax....  it all depends upon the swr and frequency, but on 75M with a 2:1 swr, you will have less than a db of loss. Not to worry.

73,
T