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Author Topic: 50 ohm antenn to 300 ohm twinlead.  (Read 2846 times)
FOXBAT426
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Posts: 274




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« on: May 06, 2008, 12:54:13 PM »

i just mounted a cushcraft ma5b on a 40ft crank up tower - it has a 50 ohm input matching box. the coax i removed from my old antenna is not long enough to make it to my radio - but i have plenty 300 ohm twinlead - the low loss good stuff with the foam. i would like to go from a short piece of coax RG8 mini(so i can connect to the MA5B) and then to a 75ft run of twinlead down to my radio. can anyone suggest a fast "balun" to match the MA5B to the twinlead and then back to 50 ohms to the radio - i do have a MFJ-949E tuner with a 4:1 balun to match balanced line to the radio - so on the radio side i'm OK. should i even mess with this or should i just go out and but 100ft of coax. will not be running more than 120 watts at this time.

thx, john
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N3OX
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« Reply #1 on: May 06, 2008, 01:31:32 PM »

  You can build 6:1 transformers but they're a little odd.  They're very much like the 4:1 balun in your rig but where it uses a pair of wires on the core, you'd need 5 wires to get close to 6:1 ratio.. actually 6.25:1 with a "quintifilar" winding ;-)

But the time you buy the right cores, wind the baluns, and buy and install twinlead standoffs to keep the twinlead away from your tower, you'll wish you just bought coax.  

Not worth the headache, I say.

73,
Dan

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

Monkey/silicon cyborg, beeping at rocks since 1995.
K7UNZ
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« Reply #2 on: May 06, 2008, 02:22:25 PM »

As Dan suggested......just get some coax.

73, Jim/k7unz
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N1LO
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« Reply #3 on: May 06, 2008, 02:34:31 PM »

Hi John,
 A fast balun would be about 6-8 turns on about 8" diameter in the remaining coax pigtail.

However, for such a short run, I'll agree that 100' of new coax is a pretty elegant solution, and keeps everything matched.

Mark
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KB9CRY
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« Reply #4 on: May 06, 2008, 06:50:37 PM »

Feed it direct with the foam stuff plus a balun as described or buy enough coax to reach the new antenna and  quit being too cheap.  Your call and your time.
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N6AJR
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« Reply #5 on: May 06, 2008, 09:46:37 PM »

buy the coax... you gave a lot of $$ for a rig, and $$ for an antenna and $ for a tower, don't wimp out on a 40 dollar piece of coax
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WA3SKN
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« Reply #6 on: May 07, 2008, 04:55:18 AM »

Well, this could become very educational!
But, the short answer is to "bite the bullet" and buy the coax!  The time involved to make it work would be better spent operating on the air!
73s.

-Mike.
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FOXBAT426
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« Reply #7 on: May 07, 2008, 02:19:33 PM »

thanks for all your great replys, especiallys KB9CRY! LOL I guess it takes one to know one!

seriously it's not about being cheap - i'll be getting the coax via the internet - it is the only way in the town i live in and i like having things yesterday -  the local radio shack has NO COAX except for 75 ohm rg59, RG6 etc. what happened to radio shack! -  i have already ordered a roll of 100 ft rg-58, but i really do like the 300 ohm twinlead - i have always had excellent results with it on my dipoles and yes my verticals! i never had a beam before so don't know how it will react to twinlead - so i'll play it safe and get the coax. thanks again!

sincerely, john
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N3OX
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« Reply #8 on: May 07, 2008, 05:28:20 PM »

"it is the only way in the town i live in and i like having things yesterday - the local radio shack has NO COAX except for 75 ohm rg59, RG6 etc. what happened to radio shack!"

Couple comments.

For *most* items, give up on local sources.  They're mostly dead.  The sooner you embrace the fact you have to buy everything online and start planning ahead  the more fun you'll have actually *working* on your projects instead of trying to buy stuff. When I have to order coax, order a bunch extra.  When I order some PowerPoles, I buy the 50 pack.  Do I need one transistor, four capacitors, and a couple values of resistor for a project?  Five transistors, twenty capacitors and a ten pack of each of the values plus a few I'm running low on to go in the drawer.

- - - - - - -

The other comment is that RG-6 is actually  decent HF transmitting coax.  It's roughly the same loss as RG-8 or RG-213, even including a 1.5:1 mismatch, actually, so it's quite a bit better than RG-58 if you can deal with the mismatch issues, which will sometimes be quite minor (more on that in a minute)

VK1OD has some info:

http://www.vk1od.net/RG6/index.htm

The copper-clad steel center conductor stuff **could** show extra loss on low frequencies if the copper cladding is thin, but my research into the particular brand of RG-6/QS stuff I bought at Home Depot shows that published loss, measured loss, and loss calculated from VK1OD's calculator for a different sort of RG-6 type cable all agree pretty well.

The mismatch is only an issue compared to using 50 ohm coax when you are trying to feed antennas that are overall a good match to 50 ohms but on the low side.  So, say, a 35 ohm antenna.

That's a 1.4:1 SWR on 50 ohm line, and since SWR remains constant along the line, you'll see 1.4:1 at the other end.  

However, *impedance* does not remain constant along a transmission line.  It goes around a circle of constant SWR as you change the line length, varying from the original resistance up to 1.4 times the line impedance.   If you use an electrical half wave or multiple, you'll see 35 ohms again.  If you use an odd multiple of a quarter wave, you'll see maybe 71 ohms.  Both a low enough SWR as far as your transmitter is concerned.  Since your rig is 50 ohms, it's seeing a 1.4:1 SWR.  Rig's happy, you're happy.

But now, hook your 35 ohm antenna to 75 ohm line.  That's a 2.1:1 SWR as far as the 75 ohm line is concerned... and just like the 50 ohm line, it varies between the original 35 ohms (which your transmitter is happy with) and 2.1x the 75 ohm line impedance... 158 ohms.

If your transmitter is like mine, it will be distinctly unhappy to work into a mismatch of more than 3:1.  So the 75 ohm coax acts as a *transformer* that takes your 35 ohm antenna up to 158 ohms. ( This sort of thing is nice in reverse, by the way, taking a 100 ohm quad loop down to 56 ohms by sticking a quarter wavelength of 75 ohm coax at the feedpoint... )

So this is the one way in which running 75 ohm coax can really mess up your SWR.  If it's a single band antenna, just changing the line length a quarter wavelength can fix this.  If it's a multiple band antenna, the best thing to do is to, at least, make the *antenna* close to 75 ohms on each band by design or with a transformer. Then the line sees a 1:1 SWR referenced to 75 ohms and anywhere you cut it your transmitter will see a 1.5:1 SWR and be happy, or you can use another transformer if you like your SWR meter needle to never move while you're transmitting ;-)

But, with the caution that you need to take this particular impedance transformation issue into account, RG-6 is good stuff for the ham on a budget (especially who needs the coax *yesterday*.  Loss of RG-8 or RG-213, most of it handles a fair bit of power, and it's what, twelve cents a foot?  Connectors are cheap too.

It's feeding a 15/12/10m antenna and my 20m/17m Moxon here.  I use the twelfth-wave transformers referenced in VK1OD's page to match the Moxon, and for the 15/12/10m antenna, I have a different (similar) series section transformer on the antenna side and a broadband ferrite cored 1.5:1 UNUN inside.  


73,
Dan








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

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #9 on: May 07, 2008, 05:36:15 PM »

By the way, the coax I have is Carol C5785 75 ohm coax.  It was purchased at Home Depot on a 500 foot spool, labeled "RG-6/QS" for quad shield.

Since there's some variation in "RG/6" it's probably good to actually specify which cable we're actually using.  

Carol C5785 has a published loss table all the way down to 1MHz if you go to their website and you'll see that the numbers are rather similar to RG-8 type ham coax.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KC8OJU
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Posts: 31




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« Reply #10 on: May 31, 2008, 07:23:21 PM »

An FT240-43 set up as 4:1 at the antenna and your 4:1 at the antenna tuner.

There's practically no losses in twin lead even at a 2:1 mismatch. Except the antenna tuner will take care of the mismatch
In other words the reflected signal will go back up and add to the output minus the loss in the twin lead which is very, low at HF.

As long as the rig sees 50 ohms you are good to go.

I went from 20 feet of coax to 20 feet of twin lead and it made my signal go up 1 S unit on 80 meters. That was only 20 feet of cable. And I was using RG8x.
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