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Dipole feed line length question

brizzotheizzo

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Dipole types of antennas are always measured tip to tip and it does not matter how wide the gap are where they meet in the middle. It is radio waves you receive or transmit that should match the ends of the dipole in a matematical relation and then you have the delay constant in the material of the antenna to consider that makes the antenna slightly shorter.

/Ubbe
My original question was not questioning the “gap” width. My question was whether the balun feed line length (black wire) was included in the total length calculation of the antenna wire (red wire) length39836508-FA97-43DF-9760-CF0C18E4366B.jpeg
 

jwt873

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I've never worried about the length of the balun pigtails... Or the actual width of the balun itself. For HF, the dimensions are trivial.

As ko6jw_2 says in his post above, the antenna formulas are a good starting point. You'll find you'll rarely get a good SWR 'out of the box' with no adjustments.

Just cut the antenna a few feet longer than the what the formula indicates and trim it back until you arrive at the lowest SWR. Cutting it longer and trimming is much better than cutting it short and then having to add wire.
 

brizzotheizzo

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So I got the antenna tuned today. Strung it up in a tree and got it all fixed up. Got it down to a 1.2 swr. Ended up not needing any sort of air choke with the coax. Noise floor was good, and got some good feedback reports with contacts.
After I got it tuned up I brought it inside and put it up in my attic. SWR was 1.1 on channel 1 and 1.35 on channel 40. Good enough for me! The noise floor went up a hair after attic installation. But nothing horrible.
 

prcguy

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Looks like you could shorten the antenna a little more and get 1:1 on ch20 and maybe 1.2:1 on ch1 and ch40. A ferrite choke is still a good idea to keep noise that can get induced onto the shield from computers and junk in the house from traveling up to the antenna.

So I got the antenna tuned today. Strung it up in a tree and got it all fixed up. Got it down to a 1.2 swr. Ended up not needing any sort of air choke with the coax. Noise floor was good, and got some good feedback reports with contacts.
After I got it tuned up I brought it inside and put it up in my attic. SWR was 1.1 on channel 1 and 1.35 on channel 40. Good enough for me! The noise floor went up a hair after attic installation. But nothing horrible.
 

Ubbe

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My original question was not questioning the “gap” width. My question was whether the balun feed line length (black wire) was included in the total length calculation of the antenna wire (red wire) length
I repeat, it's the distance between the end tip of each side of the dipole wire that counts and are irrelevant of the black wire in the middle or how wide or narrow that balun box are.

But at those low frequencies the distance to ground or other objects are usually not far away wavelenght wise and will often infuence the antennas impedance and the lenght needs to be fine tuned at the location and with the exact position that the antenna are used.

/Ubbe
 

brizzotheizzo

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I repeat, it's the distance between the end tip of each side of the dipole wire that counts and are irrelevant of the black wire in the middle or how wide or narrow that balun box are.

But at those low frequencies the distance to ground or other objects are usually not far away wavelenght wise and will often infuence the antennas impedance and the lenght needs to be fine tuned at the location and with the exact position that the antenna are used.

/Ubbe
So I got the swr’s back down. I noticed the tips of the wire ends were close to the Ceiling rafters. I increased the incidence angle at the feed point of the antenna to get the ends of the wires up a little more. That took away the SWR’s. Running at 1.1:1. I figured it was more of an interference with the antenna versus a tuning issue since it was trimmed perfect outside. Still getting about an S2 to S3 noise floor. Not terrible, but a little higher than I would like. I tried an air choke with the coax at the feed point directly under the balun. No change. The noise floor is constant and balanced on all channels, and doesn’t sound like dirty RF bleed to me. I’m not sure if an S2 or S3 noise floor is acceptable? It’s workable, I just need to possibly tinker with this thing a little more to clean it up. Definitely a work in progress. Also a bit of a science experiment. If this attic antenna idea ends up not working out to what I consider acceptable, it will go outside, irregardless to the wife’s feelings. I just need some camo spray paint to hide the balun in the tree from her. :ROFLMAO:
 

brizzotheizzo

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I have a dipole just installed in my attic. It is in an inverted V formation. However, due to the inverted V, the ends of the antenna wires come within a foot or two of electrical wire in the attic. that’s causing my noise floor to be too high. (S-4, s-5). I tested the antenna outdoors, and I had about an S-2 noise level. So it’s definitely coming from within the attic. I was going to change my inverted V to a proper horizontal dipole to get the ends of the wires away from the noise producing electric wires. The only thing that I’m worried about is ; I talk a lot to local contacts. Many of those contacts use verticals. Will I completely lose my ability to talk locally if I go with the horizontal dipole? And no, I cannot put the antenna outdoors permanently
 

jassing

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You will be able to talk to vertically polarized only if they are VERY close.
The other problem of going horizontal is you become directional, which will further impact your ability to contact anyone not in your plane.
 

prcguy

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You might look into a tuned magnetic loop, it would only be about 2ft in diameter and vertically polarized. That would fit in more places and probably further away from the power wiring. They are not that far down from a dipole in performance.
 

brizzotheizzo

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You might look into a tuned magnetic loop, it would only be about 2ft in diameter and vertically polarized. That would fit in more places and probably further away from the power wiring. They are not that far down from a dipole in performance.
Thanks, I appreciate the response. How does it perform for DX on side band?
 

jassing

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The mag loop is a good idea, there are a lot of options (some cheap, others pushing $600)
(I have never used one on sideband, so I dunno, but think it should work ok)
 

brizzotheizzo

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You will be able to talk to vertically polarized only if they are VERY close.
The other problem of going horizontal is you become directional, which will further impact your ability to contact anyone not in your plane.
Ugh.
 

prcguy

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It should work fine for locals and DX. Its bi-directional and fairly broad beamwidth but the null can be very sharp. It would be remotely tuned with wires connected to a little box near the radio and some versions run the tuning voltage up the coax. They are very sharply tuned and don't care what mode you run. A low power 100w version is no big deal to make or purchase but if your running more power they can get expensive.

Thanks, I appreciate the response. How does it perform for DX on side band?
 

K4EET

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brizzotheizzo, what is the vertical distance from the floor joist to the apex (assuming an A-framed roof)? If a flat roof, how much headroom do you have?
About 7-8 ft
With 7 to 8 feet of vertical space to work with and really needing vertical polarization, how about something like this at just under 6 feet tall:


There are some other antennas on this page:

 

brizzotheizzo

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With 7 to 8 feet of vertical space to work with and really needing vertical polarization, how about something like this at just under 6 feet tall:


There are some other antennas on this page:

So I finally figured out the cause of my s4 noise floor. When I put the dipole in an inverted “V” form I get s4-s5 noise floor. Well, I brought the antenna outside to do some diagnostic testing. The only way to get rid of the loud noise floor is to put the inverted V in a “Y” form. The noise floor goes down to S one. Transmit and receive our great. The noise floor goes down to S one. Transmit and receive are great. Honestly can’t figure this one out. I’m going to start a new thread hopefully grab some ideas as to why this is happening
 

brizzotheizzo

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So I built an inverted V dipole. I have a noise floor of s4-s5. I’ve tried it in the horizontal position and the inverted V position. Results are the same. SWR is great at 1.1 to 1.
The weird part is I decided to play with the antenna for further testing and by chance put it in a “Y” position. The noise floor goes down to S1. Talks and receives great. What the heck is going on?

and yes, I have a 1:1 balun at the feedpoint. I even tried a ferrite choke at the receiver end. No change. The only thing that fixes it is to put the antenna in a “Y” shape. Any ideas?
 

rja1

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Ok, so you have an S4-S5 noise level.........what band? day or night? QRM or QRN?

Bob
Radio Oasis
 

prcguy

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Nothing wrong with a Y shape and for some it can get the antenna up higher. Does changing shape get part of the antenna away from things that might have noise riding on it like nearby power lines, etc? You had mentioned the antenna is used indoors and you are getting what I would expect indoors, lots of noise pickup from everything in the house.
 
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