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Author Topic: 20:1 SWR on 146 MHz Vertical Antenna and 75 Ohm CATV 1/2" Hardline  (Read 3206 times)
W4JST
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« on: August 29, 2012, 07:09:52 PM »

My SWR is around 10:1 and higher in places.

I am using about 150 feet of 75 ohm 1/2" CATV hardline.

The antenna consists of about a 20" copper wire soldered to the copper clad center conductor and attached to the aluminum shield side of the hardline is 4 aluminum wire radials at around a 45 degree angle sloping down about 20" each.

Would a kink in the hardline do this or what's going on?
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W8JX
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« Reply #1 on: August 29, 2012, 07:26:39 PM »

20 inches is a bit long. If SWR is lower at bottom of band than top try pruning antenna some.
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KM3F
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« Reply #2 on: August 29, 2012, 09:49:31 PM »

First, the feedline is not 50 ohms to match at/to the radio's output impedance.
You have an impedance difference right off causing a standing wave condition or bump.
You can make the antenna match the feedline with adjustments but you have no way to measure that, at 75 ohms.
What you have left to do is keep trimming the antenna and radial angle for the lowest SWR at the radio.
This makes the whole setup a tuned system by forcing the impedance to 'look' like 50 ohms at the radio or input to the SWR meter.
Change any length even the feedline and you change the SWR.
In a system where the antenna matches  the feedline, the transmission line length makes very little difference for match at the radio if it's 50 ohms.
The only remaining parameter is the I squared R loss the feedline offers to the power applied over it's length.
Bottom line is it can be done but from a totally tuned system point of view.
We normally try not to work antenna systems that way but can't get away from it with a lot of all band antennas where an impedance transformer (tuner) has to be used to satisfy both directions.
Certainly on a beam antenna we always strive for a feedline match at the antenna that matches at the radio so no impedance transformation is needs are required.
Besides getting the TV  line maybe for free, why not use 50 ohm line and do the match properly and eliminate all the extra hassle?
Remember any SWR for any reason results in even more feedline loss in addition to the normal line loss.
So what you are trying to save by using TV line may end up not being any advantage after all.
Cable TV is designed normal for 75 ohms. Radio for 50 ohms, so trying to mix them at vhf presents issues that may make the end result worse than using an all 50 ohm system.
This is good for learning though.
Good luck.
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W8EDV
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« Reply #3 on: August 29, 2012, 11:33:01 PM »

Put a 50 ohm feedline onto the 50 ohm  radio,  and add a 50 ohm antenna, and the problem is solved....  75 ohm feedlines don't work well with 50 ohm radios....
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W4JST
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« Reply #4 on: August 30, 2012, 04:20:29 AM »

Thanks.

I tuned the analyzer from well below the 2 meter band to well above it, still about 10:1 so I don't see how a 20" radiator would be a problem. Maybe 3:1 SWR and it would get better as I went down in frequency,  but not a consistent 10:1+ SWR.

Many use this coax. The SWR should be between 1:1 and 2:1. Still not 10:1.

I measured for a short, not short.

Anyone have any ideas why the SWR is 10:1+?
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W8JX
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« Reply #5 on: August 30, 2012, 05:13:37 AM »

Well the use of 75 ohm coax would not cause problem. Double check connector on hardline and if it is good, you might try a different antenna on coax like a simple dipole as a test load.
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AA4PB
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« Reply #6 on: August 30, 2012, 05:17:44 AM »

I'm not sure what the problem is, but it ISN'T the fact that you are using 75 Ohm coax. If you make the coax length a multiple of 1/2 wavelength then the impedance at the transmitter will be the same as the feed impedance of the antenna. If the antenna is 50 Ohms then the transmitter will see an impedance of 50 Ohms, even with 75 Ohm coax.

With a 50 Ohm load on 75 Ohm coax the worst case length of 1/4 wavelength or an odd multiple yields a load impedance of around 100 Ohms (an SWR of 2:1).

There may be a problem with whatever you are using to measure the SWR not reading correctly in a 75 Ohms system. Can you read the reactance and the resistance?

Try replacing the antenna with a 75 Ohm carbon resistor. This should give you a 1:1 SWR on the feed line if the feed line and connectors are okay.
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W8JX
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« Reply #7 on: August 30, 2012, 06:14:44 AM »


Try replacing the antenna with a 75 Ohm carbon resistor. This should give you a 1:1 SWR on the feed line if the feed line and connectors are okay.


Very good tip. Smiley
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K1CJS
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« Reply #8 on: August 30, 2012, 07:19:26 AM »

....Anyone have any ideas why the SWR is 10:1+?

Sorry to ask the obvious, but have you made sure the SWR meter is good--and in adjustment? 
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KB4QAA
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« Reply #9 on: August 30, 2012, 04:19:56 PM »


There may be a problem with whatever you are using to measure the SWR not reading correctly in a 75 Ohms system. Can you read the reactance and the resistance?
BINGO!!!!!

It makes no sense to continue ignoring the impedance mismatch between the coax and antenna!!

To check the antenna, connect it to a short piece of 50 Ohm coax, (like 5 feet or so), then trim the length to resonance  and adjust the angles for best SWR (which is at 50 ohm).

Ham antenna analyzers and SWR meters are calibrated for 50 Ohm loads.  Measurements in a mixed load are not going to be totally accurate.

Once the antenna is tuned and set for lowest SWR (at 50 ohm) one would expect that the losses in you long coax to help bring the SWR down a bit.

If you want to be serious about having an efficient antenna system you need to fabricate a matching segment to bring the system impedance to 50 ohm.   The ARRL Handbook and Antenna Manual have instructions and diagrams for this.   bill
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W8JX
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« Reply #10 on: August 30, 2012, 06:50:36 PM »


BINGO!!!!!

It makes no sense to continue ignoring the impedance mismatch between the coax and antenna!!


The 75 ohm cable will cause a minor mismatch here, nothing of this magnitude. I would try a different SWR checker/bridge too.
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N3OX
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« Reply #11 on: August 30, 2012, 07:43:15 PM »

If there's a problem, it's probably not all the way out at the antenna.

150 feet of hardline terminated in nothing at all wouldn't have a SWR above 10:1 when measured by a meter calibrated to 50 ohms... 150 feet of Commscope P3-750JCA (which is a bit bigger than your stuff) has 6.4:1 SWR when terminated in an open circuit and 14:1 when terminated in a dead short on 146MHz!

A dead short will have a higher SWR referred to 50 ohms than an open circuit but in either case, you won't actually get 20:1.

I would definitely first check out your SWR meter on some known loads to make sure it's actually working.

I wouldn't expect it your meter to actually be accurate all the way to 20:1 SWR, but for what it's worth, that high of a SWR is actually impossible if the problem isn't pretty close to the shack.  Having a meter that says the SWR is 20:1 is a different matter, but actual 20:1 SWR on 150 feet of any half inch cable at VHF would only be possible if it were made of superconductor.
« Last Edit: August 30, 2012, 07:44:48 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #12 on: August 30, 2012, 07:44:05 PM »

An SWR of 10 : 1 has nothing to do with using 75 ohm coax.  That's just leading you
in the wrong direction.  A 50 ohm SWR meter will still read the SWR relative to 50 ohms
at the point that it is inserted.

With 150 feet of Andrews AL7-50 you would have an SWR at the shack of about 20 : 1
on 2m if the far end is open or shorted, or 10 : 1 if the load SWR is 20 : 1.  Most
meters aren't going to be all that accurate at such a high SWR anyway, so you can't
really tell the difference between those readings.

A simple ground plane will have a 3 : 1 SWR bandwidth of something like 135 to 155 MHz:  
the SWR won't be anywhere near that bad due to the antenna, even if it is resonant
around 142 MHz.


The most likely explanation for that high of an SWR would be a short or open somewhere,
or an SWR meter that is out to lunch entirely.

Do you hear signals on the antenna?

Have you checked DC continuity on it?  That would detect a short circuit somewhere.
Then hook a clip lead across the antenna elements and check again - if it doesn't show
a short, then you have an open circuit.

Have you checked that your radiator is making good contact to the center conductor?

Do you have an antenna switch in the wrong position?

What sort of connector do you have on the cable at the shack end?  Are you sure it
is installed properly?

Hardline can be ruined by a kink or bend, even after being straightened out, if the center
conductor has deformed the inner insulation and shorted to the outside.  It is possible
that a hard kink could cause an open circuit in the center conductor, but not as common
as just getting an impedance bump.


But your symptoms point to something like a short or open that should be easily located
with simple DC testing, or a totally disfunctional SWR meter.  It certainly isn't due to your
use of 75 ohm coax or the antenna being slightly too long.

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W4JST
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« Reply #13 on: August 31, 2012, 06:06:24 PM »

Thanks everyone. I did previously check and the hardline was NOT shorted center pin to shield. There was a bad place in the hardline somewhere though. I cut it where I saw a break in the insulation and it checked fine from there to the antenna.

I am still debating what antenna I am going to put up for 2 meters though. Want to make something. Not quite sure how to attach it to the hardline. I like the coaxial dipole or sleeved dipole idea but still debating. I would be open to suggestions.
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W5DQ
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« Reply #14 on: September 04, 2012, 04:34:38 PM »

Put a 50 ohm feedline onto the 50 ohm  radio,  and add a 50 ohm antenna, and the problem is solved....  75 ohm feedlines don't work well with 50 ohm radios....

But it works just fine if you take time to make a 50 ohm <> 75 ohm matching section and feed it correctly. Simply saying it doesn't work well because it is 75 ohms is not right ......

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
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