HFU HF Underground
Technical Topics => Equipment => Topic started by: flexoman61 on September 25, 2015, 1648 UTC
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I'm planning on making a dipole antenna for the 43m pirate band.
Each leg would be 33 1/2 feet.
I plan on running antenna across the peak of my house, laying flat on the roof.
Do I need a Balun for best performance? I'm sure I do.
If so what type, 4:1, 9:1
Thanks for any help
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I'm sure someone with more knowledge will jump in, but from what I've read, it would be best to have a 1:1 Current Balun in that setup.
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For listening? Use quality 75 ohm coax and don't sweat the balun.
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Even for listening, a balun would be advantageous. In theory, it will keep noise picked up by the shield of cable from making it to the active elements and finding their way back into your receiver. Never done an A-B comparison to see if it's worth the trouble, but I usually go by theory anyway, cheap insurance as they say. YMMV...
+-RH
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Forgive me but I'm a bit confused why would you reccomend a 75ohm coax for listening ? When the typical receiver impediments both for tx and rx is 50 ohm ?
That said a balun is exactly that a balance unbalnces transformer for matching to impedence of a dipole to its balanced 50coax counter part
Now the nominal impedance of a of a flat top dipole is 73ohms so for that you will want to use a 1:1 balun to match the impedence of your coax which is 50ohm
Your receiver is likly also going to be expecting 50ohm as well
If this is a receive only antenna I would highly reccomend som good low loss 50ohm coax and look at also some ferrite beads for keeping Rd off the shield of the coax
That's my personal opinion
Optionally you can lower the impedence of a flat top by lowering the ends of the dipole and going with a more inverted v configuration and avoid the use of having to use a 1:1 balun
Hurrah first post :)
Hope it helped a wee bit
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With a 1:1 balun, 75 ohm coax is a better match than 50 ohm for a 73 ohm dipole. Of course, chances are the only place in the universe your dipole is actually 73 ohms is in a NEC simulation.
For receiving only, matching is not super critical. I personally use 75 ohm RG-6 coax because it is cheap and easily found. I usually leave the F connectors on and use adapters as necessary.
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With a 1:1 balun, 75 ohm coax is a better match than 50 ohm for a 73 ohm dipole. Of course, chances are the only place in the universe your dipole is actually 73 ohms is in a NEC simulation.
For receiving only, matching is not super critical. I personally use 75 ohm RG-6 coax because it is cheap and easily found. I usually leave the F connectors on and use adapters as necessary.
Chris, so I can make sure I understand your advice for my own knowledge, would you advise to use the 1:1 balun in this situation or omit it?
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For lots of Antenna Info
Check
http://www.mediafire.com/folder/w8jfr02zkl8f3/ANTENNA_INFORMATION_%26_MORE (http://www.mediafire.com/folder/w8jfr02zkl8f3/ANTENNA_INFORMATION_%26_MORE)
André
CoolAM Radio - Shortwave
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With a 1:1 balun, 75 ohm coax is a better match than 50 ohm for a 73 ohm dipole. Of course, chances are the only place in the universe your dipole is actually 73 ohms is in a NEC simulation.
For receiving only, matching is not super critical. I personally use 75 ohm RG-6 coax because it is cheap and easily found. I usually leave the F connectors on and use adapters as necessary.
Chris, so I can make sure I understand your advice for my own knowledge, would you advise to use the 1:1 balun in this situation or omit it?
Personally, I use a balun with all of my antennas (dipoles or loops).
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Thanks Chris.
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With a 1:1 balun, 75 ohm coax is a better match than 50 ohm for a 73 ohm dipole. Of course, chances are the only place in the universe your dipole is actually 73 ohms is in a NEC simulation.
For receiving only, matching is not super critical. I personally use 75 ohm RG-6 coax because it is cheap and easily found. I usually leave the F connectors on and use adapters as necessary.
I don't mean to step on toes and be a ass especially because I'm "new" to your guys world but here's the thing your radio what ever it may be is expecting a 50ohm impedance by using a 75ohm coax (TV coax) you now have a loss then add your dipole which is yes in a perfect world 73ohm again there are more factors for me to list that would impact impedance of a dipole
The purpose of a balun or balance unbalance transformer is to match the impedance of your antenna to te nominal impedance of the coax going to your radio not the coax to the radio
Yes 75 ohm coax is a better match for the dipole and in that very instance a balun would not be nessicarily at all ! Other then to seperate the sheild from the antenna in which case you could use a ugly balun however the goal and objective is to Match the impedence of the radio
Again kinda superficial because you won't be transmitting
With that said things to take into consideration when using TV coax firstly it is not even close to 100% shielding like no where near 2 it has a high db loss over larger runs 3 if you use regular F connector you might as well just through a attenuator on the line then on top of the adding adapters even more attenuation
IM by no means saying it won't work im simply saying that if you want your best quality signal and you want a dipole that's ideal with low loss your best solution is to cut each legs to resonance add insulators on both ends of the less put a 1:1 balun in the center of that feed it with preferably N connectors however regular pl259 -so239 will work just fine use a good quality low loss coax with a 50ohm impedance to your radio on runs less then 35-40 feet I would just use regular rg-58u coax and then seal the outside end with butyl tape to prevent water ingress additional to cut back on some noise you can make a air wound choke with coax or use clip on ferrite beads to keep crap off the shield of the coax from near by Ed noise sources
My 2 worthless Canadian cents
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The mismatch between 75 and 50 ohms incurs an SWR penalty of 1.5:1 and a mismatch loss of 0.17dB, not enough to worry about. Each mated pair of connectors typically incurs an additional 0.1 dB.
Additionally, almost all 'satellite grade' RG-6 has a 100% foil shield plus braid which provides a pretty effective barrier against signal ingress.
I wouldn't loose much sleep over it....
+-RH
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Thanks for all the input! Wow, radio/antenna theory is certainly
a controversial subject.
Anyway here are the results. I am comparing the dipole to my 100'
longwire which goes from the peak of my house to a tree in backyard. Off the ground about 15'.
This antenna has a 9:1 balun, coax feedline (about 30'RG58-U), braid grounded to 8' ground rod.
This was just a start for the dipole, using material I had on hand. I used Radio Shack
RG58-U coax, 50ohm. Each leg is 20 AWG at 33 1/2. The downward angle of each leg is about 30 degrees.
I attached an image showing the layout. Roughly an inverted V.
I was thinking the 50ohm coax with the inverted V (lower impedance) would be a decent start.
I have a antenna switch, making it very easy to do side by side comparison. Listening to Radio Paisano this past weekend
I had a solid S9 on longwire and an S7 on Dipole with more noise. I had FLDIGI up and the waterfall show a lot more QRM
on Dipole.
So.. end result, not too impressive compaired to the longwire but does perform ok. I was hoping for a stronger signal with the
half wave dipole. Tuning around on other bands for the hell of it the dipole works well on stronger stations with about 10db less
than the longwire. For example, longwire on strong BC station was S9+10, dipole an S9.
Moving forward on this I think my next move would be Balun, hopefully cut down on QRM.
I think this comparison shows more of how well my longwire works than how poorly the dipole works.
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Flexoman, I say get that dipole up in the air, as high as possible, suspended between trees or whatever works, before you give up on it.
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Yes, I suspect that having the dipole lay on the roof is really not going to cut it.
I understand needing to use the house as a support. How about a sloped folded dipole, one end attached to the peak of the house, then sloping to ground. Make it out of strong 300 ohm TV twin lead, feed with a 4:1 balun into coax (75 ohm RG6 would be fine, or 50 ohm if that is what you have) to the radio. If you could get a mast on the roof to get that end even higher, might be better, but this would be an interesting test. If course you can use the antenna wire you have now as a first test.
One of my antennas is a sloped folded dipole, and it works great for picking up europirates.
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I don't have enough room for a sloper off peak of the house. However I have alot of trees in back yard and enough room for just about anything. The coax run back to the house would be close to 150'. Any problems with a coax run that long?
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I don't have enough room for a sloper off peak of the house. However I have alot of trees in back yard and enough room for just about anything. The coax run back to the house would be close to 150'. Any problems with a coax run that long?
Not if you use relatively good coax. I'd avoid RG58 and friends, RG-6 has less than half the loss. And you can probably pick it up for less money, and get it locally. My sloped folded dipole is way in the back as well, the run is probably 100 ft. And the run to my beverage is about 150 ft.
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Thanks a lot Chris, I like the idea of a folded dipole. That's the way I'm going to go. I looked up some reading material and came up with an article you wrote. Here is the link if anyone is interested, http://hfunderground.com/blog/?p=84
A few questions though before I start shopping for ladder line, baluns etc.
-What kind of wire are you using to attach Ladder Line to Balun? Probably doesn't matter much, but..(pic attached)
-Also, just out of curiosity, looking at images and reading up on folded dipoles the distance between the two elements is several inches instead of about the 1/4"
distance between elements of Ladder line. It's working for you so I'm not overly concerned. (pic attached).
Ladder Line will probably have to be bought online.
This will keep me busy for a while!
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Thanks a lot Chris, I like the idea of a folded dipole. That's the way I'm going to go. I looked up some reading material and came up with an article you wrote. Here is the link if anyone is interested, http://hfunderground.com/blog/?p=84
A few questions though before I start shopping for ladder line, baluns etc.
-What kind of wire are you using to attach Ladder Line to Balun? Probably doesn't matter much, but..(pic attached)
-Also, just out of curiosity, looking at images and reading up on folded dipoles the distance between the two elements is several inches instead of about the 1/4"
distance between elements of Ladder line. It's working for you so I'm not overly concerned. (pic attached).
Ladder Line will probably have to be bought online.
This will keep me busy for a while!
I just used regular 300 ohm twinlead. I bought it from one of the online ham stores, I originally used TV twinlead but it was mechanically weak, and got destroyed by the first storm. So you want something rugged.
The wires connecting to the twinlead are just regular hookup wire, and came with the balun I used.
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flexoman, this is the website i've been using in designing the one i'm gonna erect.
http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html (http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html)
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Ladder Line will probably have to be bought online.
Speaking of ladder line, especially if you transmit with 1KW or more, you can use electric fence tubular insulators as spacers for do-it-yourself ladder line, along with 14AWG insulated wire.
The insulators are available at Tractor Supply Company for just a few bucks for a box of 200.
I have successfully used the method discussed at this website:
http://www.w1aex.com/owl/owl.html (http://www.w1aex.com/owl/owl.html)
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flexoman, this is the website i've been using in designing the one i'm gonna erect.
http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html (http://www.hard-core-dx.com/nordicdx/antenna/wire/t2fd.html)
I had a T2FD for many years, it worked quite well. It finally came down after some storms. Well, most of it, parts are still stuck in a tree. But yes, an excellent performer. Mine was 132 ft long, although probably not tilted as much as it should be, due to lack of a really tall tree.