Beam Spacing

[SNAKEDR]

Good Morning Guys,

I am new to the forum and would like to ask a couple of questions about beam building techniques. I have been in and out of cb for the past 25 years. I have always wanted to build my own set of beams and recently got the opportunity. I made mine from an old set of shooting stars that fell about 100' during hurricane "Hugo" 15 years ago. I used the instructions from a set of Maco three element beams and built a set of verticle threes. I achieved an SWR of 1.2 laying flat and when I set them up verticle it went down to 1.1. I then installed the beams on a piece of 20' pipe and set them up for a trial run. A neighbor of mine that lives approximatly 3 miles away was on the radio at the time but was too close to give me a good long distance check. One thing that I did notice was the fact that I could turn the back door to my neighbor and completely cut his signal out. With the front door on him I was recieving 20+. Finally my questions are: If my antenna is rejecting that well, does that mean that it is transmitting that well also? How important is the spacing of the elements and would another configuration besides what I have work better? Any advice would be greatly appreciated.

Thanks, Mike
AKA SnakeDr 8)

 

[Beetle]

With directional antennas, there's always some degree of compromise: best forward gain; best side/back rejection; best indicated SWR...and maybe others.

Typical manufacturer's specs for a 3-element yagi would be something around "8 dB gain" (not specifying dBi, dBd, or any other reference) and 25 dB f/b ratio. I've seen yagis demonstrate right around 25 dB FBR, but at the expense of a dB or so of forward gain.

That you were copying your neighbor at what I assume you mean "S9+20 dB", and turned your antenna 180° and "completely cut his signal out" tells me that your S-meter isn't working properly (or your receiver needs aligning). A drop of 25 dB from S9+20 dB should still be somewhere around S-7 and clearly copyable.

By the way, you a veterenarian or herpetologist?

 

[SNAKEDR]

The name SnakeDr was given to me a long time ago because I liked tinkering with Cobra radios. My first wife got me personalized plates that read SNK DR and everyone thought I was a plumber (Sink Dr). My second wife changed the plates to SNKE Dr and now people think I like snakes, which I don't. Boy I just can't win. Anyway, thanks for the input.

MIke

 

[Beetle]

Ah! Cobras in the non-snake sense...sorry 'bout the mixup.

BTW: I noticed that somebody misspelled "vetenarian"...and it was in MY post

 

[W5LZ]

MIke,
Your antenna sounds about 'normal'. Playing with element spacing can improve forward gain. Usually at the expense of the front to back ratio, but that's a matter of taste I guess. As long as the 'back door' shows an appreciable difference in what you hear you're at least in the 'ball-park'. You can look around and probably find more than you ever wanted to know about element spacing (none of it absolutely solid information because where the antenna is situated plays a big part in all of it). Optomising a beam antenna is a lot of try it and see kind'a thing. Several books can give you a guide line of things to try/expect. What books and where to find them? The ARRL has some, and lord knows who else. Or invest in one of the antenna modeling program$. Some deal only with directional antennas (don't ask, I don't know).
Have fun...
- 'Doc

(They were both blonde, right?)

 

[SNAKEDR]

Hey Guys,

I took the rest of the parts from the "Hugo" incident as well as a few parts from some friendly neighbors and assembled a set of verticle threes for a friend of mine that lives about two miles away. We set the beams up exactly like mine(maco threes) and he was not satisfied. We took the (middle driven element) and moved it back towards the back of the antenna. Actually we set the driven element at 2" off center towards the back of the back door of the beam. His rejection wasn't as good as mine but the gain in transmit doubled. Does this sound right or are we barking up the wrong tree?

Thanks,
Mike

 

[Beetle]

Mike - remember: everything is a compromise when working with directional antennas. If he wants the best forward gain, the side and back rejection will suffer. If he can live with that, great. If not...not great.

 

[SNAKEDR]

Thanks for all the advice guys, I am going to keep tinkering with my set of threes until I get them the way that I want them. My friend is perfectly happy with the way that his works so we are going to leave them alone. Maybe we will get a chance to speak with you guys on the air someday. We monitor channel 31 (27.315) around here and my friends call-name is "Geronimo". Thanks again.

Mike