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Author Topic: 75 ohms cable to a vertical 80 and 160 m???  (Read 521 times)
LA4UOA
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Posts: 203




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« on: May 10, 2006, 06:18:30 AM »

Hi again.
Am going to build a vertical ant for 80 and pehaps also 160m.
The antenna will be about 135 meter away from my chack.
Have a lot of good 75 ohms coax, wonder if it can be used? or if i have to use 50 ohms?Have a  good tuner.
73 Torgeir.
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W5GNB
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Posts: 419




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« Reply #1 on: May 10, 2006, 06:45:26 AM »

Your 75 Ohm coax will work just fine.  The "Theoretical" SWR on the line will be 1.5 to 1 SWR with a 50 Ohm antenna which is not a problem, especially with a tuner in the system.

73's
Gary - W5GNB
 
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N3UMH
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Posts: 153




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« Reply #2 on: May 10, 2006, 07:35:24 AM »

75 ohm coax is typically less lossy than the equivalent diameter 50 ohm coax too.  I used 75 ohm coax on my 40m vertical for a long time; I had a 150' run.  I did end up putting a transformer at the feedpoint of the vertical to bring the shack-end SWR down.  

The feedpoint impedance of the antenna (a 40m groundplane) was about 25 ohms.  A 3:1 SWR on the line is not so much of a problem as far as loss goes, but the swr on the shack end of the coax was above 2 on the SSB portion of 40m here and I didn't want the radio to fold back the power.

I wound a 3:1 toroid transformer to match the 25 ohm antenna to the 75 ohm coax, then the swr *on the line* was 1:1.  The SWR at the shack was, of course, 1.5:1, but it was so across all of 40m and the radio was happy to put out 100W.

If you're content to use a tuner then don't worry about it at all.  I was shooting for really quick band switching by having a resonant antenna on each band and only needing the tuner very occasionally.

http://www.vk1od.net/tl/tllce.php gives some transmission line losses with mismatched loads.

73,
Dan
N3OX
www.n3ox.net
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W9OY
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Posts: 1281


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« Reply #3 on: May 10, 2006, 07:43:53 AM »

You can use any feed line you want, but you may want to do some matching at the base of the vertical.  
 
I wouldn't spend much worry on this, I would build the antennas, lay down some radials, and use the feed line you have.  You can tweak the systems performance over the course of time (add radials, fiddle with matching etc) while you have some radio fun.  It's fun to engineer a system, it's more fun to actually build something and get it on the air and work some stations.    

Hope to hear you on the air.

73  W9OY
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HA5RXZ
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Posts: 380




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« Reply #4 on: May 10, 2006, 07:44:55 AM »

Yes it will work but you have to do it right.

1) Tune your antenna so that it presents a 75 ohm load when measured at the base of the antenna.

2) Connect your coax. In theory you will now have a 1:1 SWR on a 75 ohm system.

3) Connect the coax at the shack end to an antenna matching unit. In theory you should use a 75 ohm connector but you will probably get away with a PL259.

An alternative approach is to use a balun at the shack end instead of an antenna matching unit. Transforming a 50 ohm to a 75 ohm impedance requires a 1.22:1 turns ratio so try 18 turns on the 50 ohm side and 22 turns on the 75 ohm secondary.

HA5RXZ
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LA4UOA
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Posts: 203




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« Reply #5 on: May 10, 2006, 08:25:03 AM »

Thank you all for good help,best 73 from Torgeir.
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RobertKoernerExAE7G
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Posts: 1435




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« Reply #6 on: May 10, 2006, 09:43:22 AM »

I use 75 ohm coax to my 40/80 vertical.  Its a short run, about 75 feet.  Since my amp will load into SWR of less than 3:1, I did not spent a lot of time trying to get the SWR a lot lower than that thru the 40 meter band.  
73
Bob

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N3UMH
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Posts: 153




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« Reply #7 on: May 10, 2006, 10:58:48 AM »

That's the simplest way... I've always owned rigs that reduced output power above 2:1 SWR and never had an automatic tuner or an amp, so I've always pruned and tweaked for lower than 2:1.

Unfortunately, this somewhat arbitrary limitation of 100W solid state rigs seems to creep into the collective ham "knowledge" as "you have to keep your SWR under 2:1"

It would be straightforward, but more expensive, to have an SWR power reduction circuit that would reduce power above 3:1 or 4:1 SWR and put much more robust final stages in radios.  If this were the common practice, I would imagine that whatever that number was would become the "magic SWR".

I've always had a reason to go below 2.  My new FT-857D has an agressive SWR cutback circuit that causes SSB distortion if the SWR is above 2.25:1 on my rig and the power is set to 100W!  This is a good reason to go below 2:1.

73,
Dan
N3OX
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WA9SVD
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Posts: 2201




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« Reply #8 on: May 10, 2006, 07:39:00 PM »

You CAN use the 75 Ohm coax, but you will have to "fiddle" with the matching of the antenna; to get a 1.5:1 SWR, the antenna MUST present a 75 Ohm impedence to match the feedline; then the only mismatch is at the transmitter end, where the 50 Ohm output of the transmitter or transceiver sees 75 Ohms, for a 1.5:1 SWR.  IF the antenna is matched to 50 Ohms, however, you will have a 1.5:1 mismatch at each end of the feedline, so the maximum SWR (assuming everything else is perfect) would be 1.5 x 1.5, or an ultimate SWR of 2.25:1.  Your frequency and length of feedline shouldn't alter that much, and many solid-state equipment will start to fold back at that SWR.  But if you are using a tube-type amp, it should handle the mismatch wiltout problem.  The alternative, with Solid-state equipment, is to use a tuner.
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K0ZN
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Posts: 1525




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« Reply #9 on: May 10, 2006, 10:05:56 PM »


Hi,

Realistically, the antenna and transmission line combination will work fine on the Low Bands... the only question is how well your tranceiver likes or tolerates the slightly elevated SWR. If you use a tuner the radio will be quite happy.

Another thing you could do, if it is a single band antenna is to use a Matching Stub. There are a couple of ways to do that; any good antenna book should give an adequate explanation. The only downside to Stubs is that they are a single band device.

73, K0ZN
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WA7OET
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Posts: 4




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« Reply #10 on: May 11, 2006, 06:56:32 AM »

For a 400 foot run why dont you just use open wire line since you have a tuner? Your losses will be lower compared to the coax.
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N3UMH
Member

Posts: 153




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« Reply #11 on: May 11, 2006, 08:17:08 AM »

This is NOT always true!  Ladder line is NOT lower loss all the time.  

A vertical is going to have a lower-than-50-ohm impedance.  Let's say it's 25+j0 ohms.

The SWR on 75 ohm line is 3:1
The SWR on 400 ohm line is 16:1

The matched line loss for 400 feet of Wireman 551 (window line) is 0.39dB at 3.5MHz

The matched line loss for 400 feet of RG-11/U is 1.41dB.

But the lines aren't matched!

With a 25+j0 ohm load, the loss in the Wireman 551 is 2.42dB, the loss in the RG11/U is 2.09dB.

Granted, the difference is *small* but the coax has less loss, and is going to be much easier to install.

Numbers are from here:

http://www.vk1od.net/tl/tllce.php

73,
Dan
N3OX
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