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Author Topic: In Burma now - Using 75 ohm TV coax for HF (20 mb) rig - low power  (Read 1897 times)
HS0ZIB
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Posts: 409




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« on: July 19, 2012, 01:01:25 AM »

I am now living & working in Yangon (Burma), and working hard on getting an XZ licence to operate on HF from my condo.

There is a flat roof 5 flooors above me, and I can easily run a coax feed from my condo up to the roof - and thence to an HF antenna.

I have no 50 ohm coax cable!  There is minimal chance of locating any 50 ohm cable here...

Since my rig is lowish power (10 watts to start), can I use standard 75 ohm TV coax as the coax feed?  Bearing in mind the typical velocity factor of the cable, what is the best length/multiples to use for operation on 14.070 MHz?

(If I get this licenced and off the ground, then I will fly to Bangkok to purchase 50 ohm cable and bring back my amplifier)

Simon
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KP4UFO
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« Reply #1 on: July 19, 2012, 01:16:54 AM »

Grin Grin yes you can  Grin Grin Grin Grin Grin
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ANGEL L ARCE TORRES
KA4POL
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Posts: 1908




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« Reply #2 on: July 19, 2012, 01:25:16 AM »

Yes, you can use 75 Ohm coax. You should use multiple of half electrical wavelength. The impedance of the far end will be reflected on the near end.  Thus your 50 ohm antenna at the other end of the 75 ohm will appear as 50 ohms to the transmitter.  Of course this is only valid for one particular frequency, but still close enough for the band.
SWR will be off but still sufficiently good.
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G3RZP
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« Reply #3 on: July 19, 2012, 02:01:10 AM »

In 1950s, 60s and early 70s, most UK amateurs used 75 ohm cable. The SWR on a matched 50 ohm load is only 1.5:1 anyway.
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K5LXP
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« Reply #4 on: July 19, 2012, 08:11:22 AM »

Unless there was a serious mismatch you could probably run a kilowatt depending on the exact type.  Some flavors of RG-6 actually have less loss than 50 ohm RG-8X.  The only issue you may run into is whether it has copper or aluminum braid for the shield, which translates to what connectors you end up using.


Mark K5LXP
Albuquerque, NM
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WB6BYU
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« Reply #5 on: July 19, 2012, 08:22:20 AM »

You can use VK1OD's transmission line calculator:

http://vk1od.net/calc/tl/tllc.php

Choose the type of coax (RG-6 is probably a good starting point.)
Choose "Zload" for the Mismatch type and enter a load impedance of 50 ohms.

Then experiment with feedline lengths until you get a low SWR(50).  Looks like
26m would be a good length to reach the roof.
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G8YMW
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« Reply #6 on: July 19, 2012, 10:24:50 AM »

You could always use sattelite coax, I'm sure it will be easier to get hold of, as for loss will be more than good enough
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W0BTU
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« Reply #7 on: July 19, 2012, 11:10:27 AM »

As the others have stated, you can absolutely use 75 ohm coax.

About all I've used here for many years is the CATV stuff with aluminum shielding, mostly RG-6 and hardline. And so have countless other hams.
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K7KBN
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« Reply #8 on: July 19, 2012, 11:18:14 AM »

If you use the "RG6" or a clone with the aluminum shield/foil, be sure to use crimped connectors.  Aluminum really doesn't take solder well at all.
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73
Pat K7KBN
CWO4 USNR Ret.
W0BTU
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« Reply #9 on: July 19, 2012, 11:22:41 AM »

If you use the "RG6" or a clone with the aluminum shield/foil, be sure to use crimped connectors.  Aluminum really doesn't take solder well at all.

Yes. And let me add that there is nothing sacred about soldering. :-)

Think of all of the billions of crimped F connectors in all of the 75 ohm CATV systems in the many thousands of cities throughout the world. Almost none of them go bad, unless they were installed improperly.
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