Groundplane / Beam interference ??

[Desert Rat04]

:?: I have a Jo Gunn Smokin Gun ll and a Maco V 5/8 ground plane, around 20 to 25 feet apart.. Approxamate same hight .. The guy at Jo Gunn says beam is phasing with the ground plane therefore not allowing any back or side rejection. CB buddies are killing me about this .. HA! What do you'all think ?? Is the Jo Gunn guy giving me a line ??

Thanks, Desert Rat :?

 

[RoadWarrior]

I have heard to different sides to this coin. I have
heard that Smokin Gunn II do not have good rejection.
And some say they do.
If you can, drop your Omni and see for yourself if
you are getting a line of crap or not.
Jo Gunn is not the most truthful company i do know
that much. There is no such thing as AUDIO GAIN
and there DBI GAIN figures are really way off the mark.
I say drop your Omni and see. The only real way to find out.
Good Luck!

 

[W5LZ]

Desert Rat,
Any two (or more) antennas that are fairly close to each other (as in a wave length or two) will affect each other to some extent. The frequency that the antenna is designed for (resonant and harmonics) sort of control how much 'affect' there is, the closer the frequencies the more 'affect'. Some energy radiated by one antenna is 'absorbed' by the other and that affects the radiation pattern or how well you're heard in that direction. That ~can~ affect the 'rejection' characteristics of an antenna, how much is really anyone's guess. As suggested, bringing one antenna down and checking to see what difference it made is about the only way to find out without a 'humongus' amount of information about the characteristics and mounting locations of all antennas (don't know about you, but I ain't even gonna try for that!).
Now a word about the majority of antenna companies. Only about 10% of what they tell you is ever the 'complete' and 'true' story! If that much. Exagerations that aren't totally out of reason are common (and some that ~are~ totally out of reason - LOL). That's not true for ~all~ companies/people who make antennas, but @#$ sure for most of them...
- 'Doc

 

[DXman]

I am agreeing with Doc here also.
Get them antenna's at least 1 wave length away from each other.

For instance: my beam was just 30' away from the I max 2000 and the same height, with the MFJ 259 in line and swinging the beam towards the Imax, all the reading climbed upwards. SWR went up a little and the Ohms went from 50 to 57.

Althou this is not a great significance, it will change the pattern of the beam like Doc was saying.
-----------------
DXman

 

[Master Chief]

Yep, what they said!

You MAY have interaction between the antennas. Drop one and test again. This is the best test.

---off on a tangent here---
I'd like to model that Jo-Gunn some day. I don't believe they work well at all. An interesting point though, for the "SERIOUS" 11-meter operator, Jo-Gunn recommends the SUPER AUDIO FLAT series which are nothing more than regular flat side yagis with 15KW gamma matches and upgraded mechanical and truss supports. I guess the design of the Smokin' Gun II is for the non-serious operator then. If it was a superior design, they would use it for their serious operators. The hams would use that design too. But, they don't! Why should you?

If you really want to know how well the beam IS working, drop the ground plane and point the beam at a known source. Have the source continue to transmit and spin the beam. Watch you S-meter and note the direction of the beam. When the beam is 180-degrees away from the source see if you have a significant reduction in signal. If not, then your front-to-back is horrible.

This is important, tell everyone you know, an antenna HEARS as well as it TALKS! Antennas don't talk better than they hear or vice-versa! If receive performance goes up, so will transmit performance. If it goes down.....well, you get the picture. I didn't make this up and it wasn't my idea!

Many antenna designs are scaled down to much higher frequencies and tested by receiving a signal while the antenna is rotated. The equipment will then plot out the "pattern" of the antenna. Then they transmit a signal to a receive antenna, rotate the antenna, and plot the transmit pattern. They are practically identical! The higher frequencies (in the GHz range) allows for very small antenna test ranges and makes adjustments bareable. Once they have a consistant design, they scale the antenna back down to the desired frequency and test it again.

Today, almost every antenna is designed in the computer first. Then they make the scaled models. The good antenna companies ALWAYS test in the real world and usually the computer models are pretty damn close (but not always perfect).

You can do these same tests, albeit low tech, and see for yourself! Its FUN too!

 

[sherlock]

My setup back in the mid 80's was a pdl II with a antron mounted above the pdl II with no adverse affects. They were mounted up on a 40 foot tower. I had a mast the pdl was mounted to and the antron was 9' up from that. It did not act like there was any coupling going on but while I was doing it all I heard from my friends is it wouldnt work. The beamed worked great for skip and the omni was fantastic for local. I have always heard that they should be 1 wavelength apart but I just did not have the room for that.

 

[W5LZ]

sherlock,
The reason (one of them) that you can mount an omnidirectional antenna above another antenna and not have it affect the 'other' antenna's performance much is that the omni's affect is symetrical, sort of. Affects everything equally (well, almost) in all directions., doesn't just 'interfere' in one direction so that you notice it. Uh, sort of like having a beacon on top of a tower and spot lights pointing in various directions on the tower. The spot lights still work like without the beacon on the top, the beacon just doesn't affect the 'spot' that much... (crappy comparison isn't it?) Now if you mounted that beacon in the path of one of the spot lights it would partially block the spot, right? Same with mounting an antenna near a beam, sort of...
- 'Doc

 

[sherlock]

I dont know but I thought it wasnt a bad comparison. Its funny how things work. I thought I would have more of a problem with the mast going up through the beam itself.

 

[mrcoily]

Very well put "MC" I have been working with my computer programs ..on 3 and 5 element yagis..and the spacing and gain figures can really be minupulated ,,also and especially the front to back ratios on the beams,++++ or ----..I was working on a 4 element beam optimized down to a 14 foot boom looked real good,,but after tweaking and respacing of elements i have now a 3 element with just about the same performance with a great front to back in the 32.43 db F/R ratio..with good gain in the 8.35 dbi range.. All on a 13 foot boom,,,..I have to finish my real tests but looking good...it seem that most of the beams being made now have very bad rejection,,i seem they just want to throw more elements at everybody,when they could just optimize the spacing and getter done......Anyway...73ssss Mastercheif....Dan

 

[bob85]

mc which jogunn are you wanting to model?, i hope it is the 4+4 star, years ago i had an antenna very much like the 4+4 star jogunn called a polaris commet, it was a heavy duty antenna with solid billet pegs for the elements to hang on and a heavy cast iron sadle where the jogunn is thin steel, looking at my buddies 4+4 star i would say the jogunn is a cheap copy of the polaris, once i had it dialed in my reports were at least as good as my 5 element flatside equal spaced yagi and on the vertical side it wiped the floor with my sigma4 which was mounted way higher, i have often wondered what the polaris would have worked like if the spacings had been optimised for decent gain with good f/b and bandwidth instead of the equal spacing, go to it mc model the 4+4 star i would love to know what the computor makes of it and what it would be like if it was optimised

 

[Marconi]

Hey guys, you have to be careful with the algorithm's used in even the best of those antenna modeling programs. If you read the docs carefully, you may note that there are cautions given for out of the ordinary results.

If results are too good to be true, then the old adage probably still holds true.

But do keep of the good work, because even the software developer’s don't have a clue as to where those limits really are, without further real-world testing

 

[Master Chief]

I was working on a 4 element beam optimized down to a 14 foot boom looked real good, but after tweaking and respacing of elements I have now a 3 element with just about the same performance with a great front to back in the 32.43 db F/R ratio..with good gain in the 8.35 dbi range.. All on a 13 foot boom.....

My first experience with re-spacing elements was back in the late 80's. Tests were performed on a 2-meter, 11-element Cushcraft beam (not by me). By re-spacing the elements and re-tuning the gamma match, the now 10-element beam had more gain and a better f/b. Ham radio operators have been using "long boom" beams for many years. These antennas perform better than the same amount of elements on a shorter boom. Want an example? Compare these specs.....
7-elements, 26' boom
7-elements, 30' boom
The first example has a gain of 10.5 and a f/b of 23. The second antenna has a gain of 10.9 and a f/b of 25. Of course there is always a trade off. In this case, bandwidth.

Many improvements could be made to the Maco beams I believe. Its already been done on the 104-C.

Remember though, only real world testing can prove if the model holds water!

 

[Master Chief]

mc which jogunn are you wanting to model?

The Smokin' Gun II mentioned above. Two different sets of elements. Calling them parasitic would be a stretch in my opinion.

 

[bob85]

i have made a 3 element cb beam into a 4 element optimised from the arrl antenna handbook for one of my buddies, we went from 3 eli equal spacing on 11 foot boom to 4 eli on 14 foot boom with some precision guesswork on taper schedule, extending the boom and using a 1 piece director it looked like a dogs dinner, with a small increase in gain a big improvement in f/b and better bandwidth you could almost hear the smile on his face when he called dx into the states without getting hammered by russians on his back door

 

[Devildog]

I had a skyking 2 element quad with my I2K mounted above it with no interferance I could tell. I've talked to Dxman on it before some strong winds took care of it Now I a have a couple more months before I can get the base back up. It will be with no beam though.