|
|
|
|
|
INTRODUCTION TO K7ITM'S
ARTICLE
The first gain antenna that I built was a coaxial collinear pieced together with half-wave sections of RG-8. I didn't do anything other than guess at the velocity factor of that coax and cut the sections accordingly. I put this string of halfwaves into a fiberglass pole that I had and leaned it up against an elm tree in our yard. It was great! With my Pawnee (10 watts AM) I worked all over. Eventually the antenna gave up the ghost and had to be replaced. Years later I decided to build another for 220, but now I had read about problems with encasing an antenna inside a PVC tube. It changed the frequency according to readings from the SWR meter. So I made a new coaxial antenna. The SWR said that I was way off in frequency so I spent too much time trying to get things the way the meter said that I should. Nothing made sense. If only I had read and understood the article that Tom Bruhns, K7ITM, wrote below, I would have saved a lot of effort.
WA8MFL
COAXIAL COLLINEAR HINTS
FROM K7ITM
A few years ago, I spent quite a bit of time learning about and understanding the coaxial collinear construction, and hope that what I learned can benefit others:
1. Each section must provide 180 degrees phase shift. That is, each must be 1/2 wave long, taking into account the propagation velocity of the coax. With lossless line, then the _voltage_ between the ends of the radiating (outer conductor) part of adjacent elements will be in phase and equal amplitude all along the antenna. What you really want is in-phase antenna current in each element, which in-phase voltage does not guarantee, but the simulations I've done indicate that the currents are all very nearly in phase, given the equal inphase voltages. The current magnitudes are low in some elements of long arrays, but that just means that those elements don't contribute much to the overall pattern; the important thing is that their currents are still in phase, so they don't detract from the pattern. Practical coax has low enough loss to do a very good job making the voltages
equal.
(This supports the measure carefully/construct carefully advice.)
2. You are essentially end-feeding a bunch of elements. The phasing system puts the feedpoints electrically in parallel. (Remember: the load impedance you put on the end of a half-wave piece of coax is echoed back to the feedpoint. In this antenna, start at the top with one impedance, echo that back 1/2 wave and parallel it with another impedance...) When you have few sections, the net parallel effect is still a fairly high impedance. (You can't just say you're paralleling equal resistances, be
|
|
|
|
|
|
|
|
|
|
cause there is mutual coupling among all the elements, but in a qualitative sense, it's what's happening.) So the feedpoint impedance at the bottom of the antenna will be probably in the 400 ohm region with just 3 or 4 elements. With 10 elements, it drops to the 150 ohm region. Whatever it is, you must match that to your feedline if you want low SWR. For one-off antennas,
I like to use an "L" matching network, with a coil in series, and a trimmer cap shunt across the antenna. (Note: you can feed this antenna at the end, or across the connection between any pair of elements. Remember, 1/2 wave sections just echo the impedance.)
Also note: the elements are considerably less than 1/2 wave long. The half-wave is taking the coax velocity factor into account, but as radiators, they are only about 0.66*1/2 wave (if you use solid poly coax) or 0.8*1/2 wave (if you use foam dielectric cable). So they are NOT RESONANT. That is NOT a problem. Non-resonant antennas radiate just fine; it's just that their feedpoint impedance is NOT a pure resistance. So...the matching network must also take out some reactance. Again, an "L" matching network does this just fine. Do NOT expect a simple 1/4 wave of 75 ohm line (like shown in the ARRL writeup) to do a good job in all cases.
3. As with any antenna, you must be sure the feedline is decoupled from the antenna to assure the antenna pattern remains as you designed it. You can do this with radials, sleeves, chokes, ... but especially with a high gain antenna, it's an important part of the design. I'm finding that chokes (coils of feedline) at the base of the antenna and another a quarter wave or so lower work well. But other ways work too.
Summary: Get the phasing right by making the sections an electrical halfwave...use careful construction. But errors of 1% or so won't kill you. Match the resulting impedance to your feedline: don't worry that the antenna is not "resonant". Decouple the antenna from the feedline.
If you divide the problem into these three separate items and get each right, it will work for you.
(Note: if you do it this way, you can see that putting it into a
"radome" of fiberglass or PVC pipe, so long as the pipe isn't too lossy, will change the impedance (requiring a change in the matching network tuning), because it changes the mutual coupling among elements, but it doesn't materially change the phasing, because that's all done in the coax electrical length. And the antenna gain is critically dependent on the phasing, not on the particular feedpoint impedance.)
(Presented with permission of Tom Bruhms. K7ITM.)
|
|
|
|
|
