[VHFcontesting] [VHF] Another antenna question

[Alex]

Hi Jacob,
 
> After asking the overall evaluation of the current loop antennas, I am
> thinking more along the lines of the KISS system for my mobile/rover
> setup.(Keep It Simple Stupid) 

I think that's an excellent approach to many things. :)

> Therefore I am now thinking about running a simple Hamstick dipole setup for
> 6 meters since a loop is nothing more than a dipole with the ends folded
> around into a loop/square.

> It should work as well as a loop, shouldn't it?

If you are able to rotate the dipole (armstrong method perhaps), then yes, it works a heck of a lot better especially since you can get it up 20 or 30 feet.

I used a dipole (made from aluminum tubing) at 16 feet and it worked very well. Since the pattern of a dipole is rather broad, you don't have to turn it as much either. Turning it once 90 degrees and you have 360 degrees coverage.

Someone else pointed out that a loop has a radiation pattern within a dB all around, but that is not really true. In the case of a halo close to the car you can indeed achieve this, but that is only because most of the energy is radiated upward. It's like pointing a flashlight upward and then hold a piece of paper above it and then conclude from the reflection you see on the paper that the radiation pattern is nearly perfectly round (which is caused by the reflector in the flashlight, or the car body and ground that act as a reflector in the case of the antenna just a few feet mounted of the car.) 

I have found that with single and stacked loops that the radiation pattern more that of a peanut when properly elevated. This is also discussed in the RSGB VHF Handbook. Front to side difference in gain can easily be 3 dB or so. Now if you consider that a loop already has considerable less gain than a dipole, and then if you'd reduce the gain of the halo even more by mounting it closer to the roof of the car, then yes, your proposed setup will be much much better. It really bocomes more like comparing apples and oranges.

If you really want a loop with omnidirectional characteristics, then you'd have to build a full wave loop. See RSGH VHF Handbook.

During the June VHF QP I had a 6m halo about 4 feet from the car roof, and a 2m 5/8 wl vertical that I used as a 1/4 wl on 6m. For close in contacts the halo was only marginally better than the vertical surprisingly enough. You'd think that the 20dB or so polarization loss would make a huge difference, but I guess the gain loss I experienced by having the halo so close to the car roof actually made the vertical a decent competitor. For the short Es openings that we were afforded by the propagation gods the vertical worked so much better (not going to quantify this w/o measurements) that I concluded to forget the halo next time around, unless I can get it at a decent height. Keep in mind that this is just a one person experience. Maybe others would have better results.


Hopefully this was confusing enough. :)

73,
--Alex KR1ST
http://www.kr1st.com
http://www.airlinkexpress.org