Help with coaxial collinear antenna - elmers Forum at eHam.net

$\'\'$

$\'\'$

$\'\'$

eHam.net Forum : elmers : Help with coaxial collinear antenna

Forum Help

1-7 of 7 messages

Page 1 of 1

Help with coaxial collinear antenna	Reply
by K6CMJ on May 1, 2008	Mail this to a friend!
I posted this on the VHFUHF forum for a few days and received no definitive answer. I'm going to build a coaxial collinear for 2m/70cm, and need help with the design of the top 1/4 wave portion. I've seen several designs, all are similar, except for the top section. I've seen three types, and need to know which is correct, or better, or if there's no difference etc. The three types I've seen are: 1. 1/4 wave whip attached to the center conductor of the portion below. 2. 1/4 wave whip attached to the center conductor and shield of portion below (shorts them out). This is the most common I've seen. 3. 1/4 wave coax section just ends, no whip, no shorting. Any help is appreciated. Chuck

RE: Help with coaxial collinear antenna	Reply
by M0JHA on May 2, 2008	Mail this to a friend!
hello, youll find this with many designs especially off the net.. my advice would be clip a quarter wave piece of wire to the top using a crock clip or something similar , that way you can clip it in the differant ways and see which works without commiting yourself to solder.. the good thing with doing this is you will learn a little about the design instead of simply just following instructions.. as far as which is correct i dont know, i did start making one of these coax antennas but got tired with messing with bits of coax and as it was simply only to make for the sake of it and not to be used gave up.. billy uk

RE: Help with coaxial collinear antenna	Reply
by W8JI on May 2, 2008	Mail this to a friend!
Chuck, Most of the designs for coaxial collinear antennas are very poor designs. They are very poor copies of Phelps Dodge's Station Master series. I've built several from Ham articles and they have never worked as claimed. In order to work properly, the velocity factor INSIDE the cable must be the same as the velocity factor outside the cable. Phelps dodge did that two ways. In some antennas they potted the strandard coaxial cable by filling a fiberglass radome with wax. The wax was rather thick (a few inches) and had the same velocity factor as the polyetelyne coax inside. The second way they accomplished that was using bare shield uninsulated air dielectric cable. That cable was fitted into a hollow radome, so the velocity factor outside the cable was the same as the velocity factor inside the cable. I'm not sure how all these articles ever got an antenna to work because unless the wave travels the same speed inside the cable as outside the cable, they will not work as planned. 73 Tom

RE: Help with coaxial collinear antenna	Reply
by KD8HZC on May 2, 2008	Mail this to a friend!
I also attempted to build one from some RG-8 I had. Getting the lengths cut and tinned wasn't a big problem. Nor was getting them hooked together. It was trying to keep em all dead straight.I was tacking them to a dowel rod with hot melt glue. But for me the hardest part was figuring out how to decouple it at the bottom. Not having a machine shop it was pretty ugly looking. By the time I figured in all the velocity factors, cost of some sort of radome, a mount that would hold up. I gave up and bought one. But please don't give up trying it out. I did learn a few things along the way. I have very rudimentary test equipment. A VOM and a Field Strength meter, and a calculator. For the top radiator, I used a welding rod soldered to the center and shield. Bob

RE: Help with coaxial collinear antenna	Reply
by K6CMJ on May 2, 2008	Mail this to a friend!
Thanks, I'll keep those points in mind, and also try the experiment with the jumpered whip.

RE: Help with coaxial collinear antenna	Reply
by N3SDO on May 8, 2008	Mail this to a friend!
Hi Chuck You said originally a 2m & 70cm ant. Are you trying to build a dual band ant, or 2 seperate ones? I have made several un-successful attempts to build a single band coax collinear for 2m, and an miserable failure just last night on one for 70cm made out of 9913 coax. Yes, the inner/outer velocity of normal coax appears to screw up the 1/2 wave phasing relationship. I started tinkering with pieces of phasing triple hairpin loops from an old MFJ Hyper Mighty gain dual band ant, the was a band soldering intermmitent nightmare! Doing some reverse engineering and finding some very odd phasing and element lengths. I have build J-poles, double Js, Ground planes, 5/8 gp, 1/2 phased over 1/4, but I can't build a coaxial collinear that works! I had an idea thismorning, a 1/4 wave ground plane, with a 1/2 wl phasing hairpin feeding a 1/2 wave radiating element. \| \| \| === \| What if the 1/2 wl phase is a 1/4 shorted stub of coax stuffed up inside a 1/2wl brass tube, insulated at the top and shield soldered at the bottom of the tube. \| \| \| \| \|\|\|\|\| < shorted 1/4 coax stub \|\|\|\|\| < shield soldered to tube bottom \| \| < coax fed brass / copper rod ground plane. In my mind, this thing could be stacked, and brass tube made telescoping in segments, and tweaked to work much better then the fixed length coax segments... To be continued... N3SDO

RE: Help with coaxial collinear antenna	Reply
by N3SDO on May 9, 2008	Mail this to a friend!
Hi Chuck I've found some notes on the net from other builders that have had limited success adjusting the swr on these designs. The 1/4 wave section on the bottom often doesent provide a correct match. Also when you slide it into a fiberglass radome or pvc, it will change the resonance also. An L/C tuner type network may be what you are looking for to match it up. Good luck! I think some of these guys build something that kinda works so-so and then write a magazine article as the ease of building this wonderfuld device. I've fallen for these things myself... Sometimes they work, sometimes they dont. Always a learning experience. Ed N3SDO

Page 1 of 1

Next Topic: Tone Generator
Previous Topic: New High School Club?

Email Subscription
You are not subscribed to this topic.

Check our help page for help using Forum, or send questions, comments, or suggestions to the Forum Manager.

$\'\'$