[VHFcontesting] Handling coax mismatch--another way

[Marshall Williams]

Hello Chet and everyone else on the list....I would add another tried 
and true method....a matching stub.  You can place a T connector in the 
feedline.  The matching stub goes to one side of the T, the antenna to 
another side of the T, and the rig to the last side.  The matching stub 
should start out just a bit longer than 1/2 wave length(counting the 
velocity factor) and open on the far end.  Then you start trimming.  I 
have found that at some point, you "get it right" and the SWR goes very 
flat. I know that this is not "scientific", it just seems to work.  You 
can put an MFJ antenna analyzer on the rig side of the T and watch the 
SWR as you trim.  It will just come right down when you get close to the 
"Magic Length".  I have done this on 6M, 2M, and 432.  I use RG-58 size, 
teflon insulated coax for this, but you could use LMR-400 or just about 
anything you have.  You would not want to use very lossy, old, water 
filled coax for this.  There is little power "used up" in such a stub. 

There are guys that can calculate exactly how long such stubs should 
be.  I am not one of them.  However, they first put their $50,000 
analyzer on the system so that they can get the exact parameters they 
are trying to match.  The worst that I do is to burn up a few feet of 
coax. 

Again, I am not a genius on the theory side of this, but basically, the 
stub has reactance--either Xl or Xc.  When you get the trim just right, 
the reactance of the stub cancels out the reactance of the antenna / 
feedline.  That may be too simple an explanation, but it works for me.  
When you get the trim right, just waterproof the open end of the coax 
carefully and you are done.  There may be VERY high voltage at the open 
end, so be careful there.  I usually trim just a tiny bit of the shield 
back so that there is a longer path from the shield to the center 
conductor and put a good quality heat shrink boot over the end--then a 
couple of wraps of Scotch 88 vinyl tape.  Clearly, this is a single band 
solution.

I welcome better explanations from the guys qualified to do so.....

73 Marshall K5QE

Chet, N8RA wrote:

>In case I am not the last person on the planet to know about these
>techniques, let me describe a recent station improvement.
>
>My situation was the use of 75 ohm hardline with 50 ohm antennas. With a
>pure 50 ohm antenna, the resultant swr at the station end of the hardline
>can be as high as about 2:1, but it can be even higher if the antenna was
>not truly 50 ohms resistive.
>
>At HF, it seems common to just live with the additional SWR variations that
>are introduced. As long as your transmitter or amplifier can deal with it,
>the additional losses due to SWR in the hardline are not very big. 
>
>But in my case, I have a couple of extra consideration:
>
>>From my 6M antenna system, I was seeing an SWR at the transmitter as high as
>2.7:1 from what I thought was a fairly well matched yagi. The calculated
>additional line loss introduced by this was starting to get over 0.5 dB,
>which for weak signal VHF and above is getting to be of concern.
>
>An even bigger problem for me was from the fact that when I would switch to
>a different 6M antenna, the SWR in the shack from it was close to 1.2:1. If
>my amplifier was tuned for antenna 1, it didn't like antenna 2 too well, and
>vice versa. So I really needed to tame the wide SWR (impedance) variations
>being caused by the use of the 75 ohm hardline.
>
>A web search showed a number of ways to do this.
>
>1- use a ferrite transformer wound for the ration needed. I chose not to do
>this due to concerns with stray coupling paths and properly terminating into
>coax connectors, (plus I did not have any suitable cores in the junk box).
>
>2- use a quarter wave matching section employing a coax cable having an
>intermediary line impedance. The needed impedance coax was not in the junk
>box either so this would have meant building something using proper inner
>and outer diameter tubes and pipes to get the impedance right. And at 6M,
>this might be rather big. 
>
>3- use a twelfth wave matching transformer. This technique uses nearly
>quarter wavelength long pieces of 50 ohm and 75 ohm coax cable joined in the
>center. Hmmm, I just happen to have a box with some short pieces of 50 ohm
>RG213 and 75 ohm RG11, and they just happened to have a PL259 connector
>already on at least one end. I could have just cut these to length and added
>connectors and joined them with a coax barrel, but I chose to splice them
>permanently together and avoid the middle connector joints. I used a ½"
>length of brass hobby tubing and soldered ¼" of each center conductor into
>it, then packed around it with some pieces of coax dielectric material, one
>tape wrap to hold them, and then soldered overlapped shield braids coming
>from each side. Initially cut your coax pieces an inch too long, strip back
>the outer jacket, fold back the braid, and then cut off the extra inch. This
>leaves you with a nice long braid to fold back to the other one. At the
>finish, I wrapped the whole joint with tape, laid a long brad nail across
>it, and wrapped again. The brad keeps the cable from being kinked at the
>relatively fragile center junction.
>
>
>I made two of these transformers, one for each end of the hardline. How did
>they work? Great! Now the different antennas each show an swr close to 1.4:1
>and their impedances are within a few ohms of each other. Be aware that
>these are single band devices, which in my case was no problem since this
>run of hardline is dedicated to the 6M antenna system. 
>
>Credit for this approach goes to Darrel Emerson, AA7FV, and you can see his
>details on how to do it at
>http://ourworld.compuserve.com/homepages/demerson/twelfth.htm. He also shows
>how to use the two match phased antennas. 
>
>73,
>Chet, N8RA
>
>  
>