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Author Topic: feeding HF Dipole  (Read 5894 times)
YONU
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Posts: 67




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« on: December 25, 2012, 06:29:26 PM »

I am in the process of building my first HF Antenna and need advice I am building a simple dipole for the 10 meter band however what I was considering doing was running 300ohm Twin-line down the mast(PVC) to a Balun that changes over to Coax I wast wanted to see what everyone thought of the idea.

Thanks,
Reg
KK4NJC
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WX7G
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« Reply #1 on: December 25, 2012, 06:45:17 PM »

Your dipole can be fed directly with 50 ohm coax. A 1:1 current balun is optional.
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N4CR
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« Reply #2 on: December 25, 2012, 06:50:03 PM »

Skip the 300 ohm section and feed it in the middle with coax. A 1:1 current balun at the feed point is optimal but not mandatory.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W0GXA
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Posts: 27




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« Reply #3 on: December 25, 2012, 07:06:38 PM »

Google "ugly balun".... simple way to decouple the feedline from the antenna.  That's what I use on all my dipoles.  Note: it can't hang directly against a metal support (as in the case of an inverted V).

I have a couple pictures on QRZ.com/W0GXA

Good luck

73, Bob
W0GXA
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W5WSS
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« Reply #4 on: December 25, 2012, 07:19:30 PM »

Yes feed it with coaxial cable. I agree, Locate a good 1:1 current balun Preferably up at the feedpoint that incorporates a good strain relief system and antenna wire designed for rigorous conditions and weather where tensions applied are compensated better such as the brand DX Engineering attachments. A good PL-259 such as Amphenol Brand.

And

An insane amount of weatherproofing material around the Pl 259 connection.
These are things that will lengthen the reliability and enjoyment of a successful installation.
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WN2C
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Posts: 454




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« Reply #5 on: December 25, 2012, 07:40:19 PM »

If your antenna is for ten meters only, feed it with 50 ohm coax not TV twin lead.  If you are making a longer antenna (single wire) and want to use it on other bands you can use twin lead or 450 ladder line.  But...and this is a must... you must use an antenna tuner or coupler.  This will help you match the impedance of the feed line to the transmitter.  The reason you don't run any kind of wire line (like TV twin lead or 450 ohm ladder line)  down a metal mast is you end up coupling the line to the mast and you won't be able to tune.  If you want to use twin lead keep it away from the mast or other metal objects by at least 4 to 6 inches and use a tuner with a 4 to 1 balun.
73
Rick  wn2c
 p.s.
re-read your post and saw the PVC - that's OK to run it down.  But I ask how high is the dipole going to be using PVC as a mast or support (center support?)
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W5DXP
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« Reply #6 on: December 25, 2012, 08:01:35 PM »

But...and this is a must... you must use an antenna tuner or coupler.

He could make the dipole 1/2WL long and make the twinlead an integer multiple of 1/2WL long. The twinlead becomes a tuned feeder that repeats the antenna impedance and fed through a 1:1 balun-choke, doesn't need a tuner. Assuming the twinlead has a velocity factor of 0.9, 1/2WL of it on 28.4 MHz would be 15.6 feet so 31.2', 46.8', 62.4', or 78' would be good lengths of twinlead for avoiding an antenna tuner.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
YONU
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Posts: 67




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« Reply #7 on: December 25, 2012, 09:25:06 PM »

here was my reason for the twin load and tell me if this is just wrong thinking in order to get the feedline from the radio to the antenna I have to bury the cable I have enough coax on hand to goto the base of the center mast for the Dipole and I figured since I had a roll of Twin-line I would run it down from the antenna to the base of the mast as I am using a PVC mast.

Is this a bad Idea?

Thanks,
Reg
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YONU
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« Reply #8 on: December 25, 2012, 09:36:20 PM »

oh and I do pan eventually to all lines to the dipole for other bands
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KM3F
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« Reply #9 on: December 25, 2012, 09:59:36 PM »

The center feed dipole at normal height is about 72 ohms impedance.
50 ohm coax is so close that you trim the dipole ends to get the match to less than 2 to 1 or the best you can make it..
300 ohm feedline is to high and far away from the antenna impedance.
The SWR would be around 6 to 1 with twin lead spliced to coax or some other value you don't need.
That's not what you want to end up with at the radio after you hook up coax to it.
A balun won't even take care of this if you use Twin lead.
The only other way is to make the antenna a folded dipole.
That config is close to 300 ohm  to fit the twin feedline but you have to keep it spaced away from ground and other influences and still have to transform to 50 at the radio.
Better to splice the coax with another piece and extend it to the antenna.
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M6GOM
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« Reply #10 on: December 26, 2012, 04:17:28 AM »

If the mast is metal you can't have the twin lead running vertically anywhere near it.
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AC2EU
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« Reply #11 on: December 26, 2012, 05:26:55 AM »

I feed mine once a day.  Grin
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YONU
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Posts: 67




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« Reply #12 on: December 26, 2012, 07:25:30 AM »

ok first I have to ask what do Dipole like to eat I want min to grow-up to be big and strong  Wink

ok you said that the impedance of the dipole at normal height is 72 Ohm could I run RG6(75ohm) up the mast and connect the leets to it and at te bottom of the masy have a 50ohm ugly balun to bring it all back to 50ohm?  sorry just no one around me local sales 50ohm coax other than RS and they only hace RG58

Thanks,
Reg
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AC2EU
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« Reply #13 on: December 26, 2012, 07:43:57 AM »

ok first I have to ask what do Dipole like to eat I want min to grow-up to be big and strong  Wink

ok you said that the impedance of the dipole at normal height is 72 Ohm could I run RG6(75ohm) up the mast and connect the leets to it and at te bottom of the masy have a 50ohm ugly balun to bring it all back to 50ohm?  sorry just no one around me local sales 50ohm coax other than RS and they only hace RG58

Thanks,
Reg

That would work fine all the way to the shack. In fact,it's a better choice than 50 ohms! Your antenna tuner will do the rest. No need for a balun at all unless you want a 1:1 current balun at the antenna end for current distribution, but that is optional. If you are using more than 100W, check the power/freq specs for RG6, I'm not sure what it can take... 

BTW: My dipole eats RF, lots and lots of RF.   Roll Eyes
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W5DXP
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« Reply #14 on: December 26, 2012, 07:58:46 AM »

Is this a bad Idea?

For single band operation, you can run one of the "magic" lengths of twinlead that I calculated above (using insulated standoffs) down the pole to the coax/twinlead junction where you need a 1:1 choke between the twinlead and the coax. In general, when one has an SWR of 1:1 on the coax, any length of coax can be replaced with an integer multiple of 1/2WL of parallel feedline. Just remember to keep the parallel feedline a few inches away from any conductor. I use electric fence insulators.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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