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Author Topic: New installation and SWR observations  (Read 1181 times)

Posts: 756

« on: June 25, 2009, 04:57:09 PM »

Hi. I FINALLY got both of the 95' towers up. Last weekend (missing the VHF contest by a week, natch), I connected the antennas and feedlines on tower #1.

This is all 'for what it's worth', so if you are not interested, please don't flame me!

Tower #1: 6 meters
Modified M2 6M7JHV at 100'. I used new 9913F7 to go from the antenna's N connector to just below the rotor. Here, a N male/female junction was made using a homemade adaptor to Commscope 7/8" CATV hardline.
Why the CATV stuff? 'Cause I have thousands of feet of it, brand new, still on the spools. It has low loss and will not asorb water. It ia also easy to work with, and light....
Anyway, at the bottom, I drilled holes in the house and brought the hardline into the shack. At this end, I also used my connector and an N female. From there to the radios, I use coax (to be discussed in a minute).

Same tower: 2 Meters
Carefully rebuilt KLM 16 LBX at 115'. (one QTH had a pair of these stacked).
Again, new 9913F7 to just below the rotor. There I have a THP mast mounted Preamp. The THP mounts directly to another run of the CATV stuss via a 'double male N.
Same jumper scenario in the shack.

Soooo, today I did this simple test:
(first of all, I am NOT a worshiper of 1:1 SWR. I'll leave that to the CBers. ha ha)
I took 2 pieces of coax , random length, already made up, but long enough to go from the CATV bulkhead to the main operating table. One piece was a section of older 9913 and the other was RG-11, both Belden.

First I looked at the 6 Meter SWR with my MFJ 259B.....

Using the 9913, the lowest SWR on the JHV was 1.4:1 at 49.7 MHz.
OK, I have had that reading at one of the last QTHs..
So, I looked at the SWR at 50.125. It was 2.2:1.
Next, I put the RG-11 in place (you know, 50 Ohm radio to 75 Ohm feedline to 50 Ohm antenna, you get the picture.....)
Lowest SWR freq changed slightly to 49.82, but still 1.4:1. Next, I measured the SWR at 50.125...2.2:1.

On 2 Meters, the results were more dramatic:

RG-11 Jumper: Lowest SWR (2 nodes) 142.5 / 146 MHz = 1.1:1.
SWR at 144.200 4.3:1 ouch!
9913 jumper: Lowest SWR (2 nodes) 141.1 / 148.7 MHz =
SWR at 144.200 2.1:1.

Believe me I KNOW I could prune the jumpers to get a 1.1:1, but it ain't worth it.

On 6, my TR-6 will load into a wet noodle and the auto tuner in my 756 PRO instantly knocks the SWR to 1:1.

On 2M, my SS amp should be quite happy with the 2.1:1 SWR......

I just wanted to share these results with you fine folks.

At one time, I thought about using sections of transmission line as imp transformers, but I figgered ALL those connections are just an invitation to losses and moisture ingress. After all, I am in the "Sunshine State" ha ha

I do not believe that an amateur on either end could 'detect' the difference of a 3:1 or 1:1 SWR.

thanks fer the bandwidth!


Posts: 21764

« Reply #1 on: June 25, 2009, 05:24:20 PM »

I wouldn't worry about SWR either, other than to check out if the mismatch is being caused by a bad connection.

However, you have a lot more guts than I do putting preamps up on 100' tall towers!  Those damned things always failed for me when I was in lightning-prone New Jersey, and always required some service, probably at least once a year.  Not so bad if the tower's short but if the tower's tall it becomes a PITA pretty fast, hi hi.

Congrats on the install, I'll look for you on six!


de WB2WIK/6 DM04

Posts: 817

« Reply #2 on: June 25, 2009, 09:01:42 PM »

With 2 high towers and all the work involved,
why not do all the testing of the feedlines on the ground with a good termination to see what you have first.
Going to those lengths for low loss max signal, big antennas and very nice towers should be a prime requirement to test out first and not last with the possible need to do it all over again.
I have just completed a 150 foot run with LMR 600 feeding 7/8" Heliax and back to LMR 600 to the antennas for 6 and 2m thru a coax relay yet..
The vswr is less than 1.2 on 2m. The total line loss is less than 1db.
The match was all tested ahead of time with an absolute flat termination on 2m.
No doubt your system will work good but with that kind of effort you should be able to do better.
Wish'ed I had 1 100 foot tower.
Good luck.

Posts: 2415

« Reply #3 on: June 26, 2009, 12:34:29 AM »

Congratulations on building a good working system on a fair price tag.

I am a strong proponent of using surplus CATV hardline. Super low cost, AND super low loss!

I do not agree however with Belden 9913.  We have had so many problems with several "versions" of it we gave up on that stuff and use TIMES LMR 400 instead.

Anyhow, I too do not get all bent out of shape if there is a little reflected power.  What counts is the overall performance of the entire system.

Posts: 756

« Reply #4 on: June 26, 2009, 04:26:27 PM »

Thank you for all the nice replys. I DID consider LMR, but there was some concern in my research about the connectors. The 9913F7 is good stuff.
I have a machine shop here and make my own adaptors  from 6061-T6 billet, to transition from the CATV stuff to the coax. (MY adapter version #3) ha ha

I have tried matching the 6M7JHV 'on the ground' at the last 3 QTHs it was installed. I could NEVER get the SWR to where it should be. Yes, I tried 'pointing it up', 'getting it as high as I could', etc. I even replaced the coax balun this time, since measurements indicated it had deteriorated, and shifted frequency. Same result. Yes Steve, I used the waterproof 'F' connectors. And yes, I KNOW that the Dish companies use them, but I still say, "give me an "N" connector any time". I would rather put together 10 'N's versus 1 PL-259!

Yes Steve, I have had LOTS of preamp failures, back in Kentucky. Now that I am close to the USA lightning capital, I have prepared for that. Plus, I'm getting good at fixin' them!

If anyone needs the preventative measures, contact me directly. Oh, I also have spare preamps for each frequency and my installation requires minimal messing around to swap them out...... Plug and play, if you will.
Believe it or not, at the 100' level, there are still a couple of taller trees (in one direction) that slightly block my view to the horizion. We got some tall a$$ trees here!

I'll be on 6M Scatter tomorrow AM (if the thunderstorms will allow it!!) ha ha


N4UE EM80pa
ex N4KCM

Posts: 2415

« Reply #5 on: June 26, 2009, 10:47:53 PM »

The cheap, Easy way to get connectors for cable TV hardline is to simply adapt cable "Pin" type connectors to a PL 259.

(These are the very common connectors that go from the hardline to all actives and passives- Just a "pin" sticking out the end.)

Cable splicers toss LOTS of them in the dumpster when they cut the pin a little too short. THOSE are the connectors to get. (Or some used ones)  You only need that center pin to be about 5/8 of an inch long to work for ham use. (For cable use, The pin needs to be a few inches long, Depending on just what it is going in to.)

Take a PL 259 apart, Cut the outer sleeve about in half. Note that the thread is the SAME as the pin connector!  That gives you your outer part.
Pry the center pin out of the PL 259 connector and solder it to the pin of the cable connector.  
DONE!   Screw in a feed through to connecto another PL 259 to your new assembly!   Works great.

Bring a bag of donuts etc for the cable TV guys, You should be able to get lots of the common pin connectors.
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