The AM Forum

So here's my conundrum:

I don't have quite enough yard space for a full length (60 ft each side) 80/75m dipole, but of course I want one. My newly acquired DX100B needs to have access to 80 M. What I have now is a 40M dipole, inverted V, apex at 30' on a hex beam push up mast, running down to two 10' aluminum poles at each corner of the yard. I have a run of 58' available on one leg and 56' available on the other. So the 40M dipole fits in the space easy-peasy. An 80M dipole at 60" each leg will not quite make it. Here is what I would like to do:

Same inverted V, same apex at 30' feet. I run the 80 M legs to about 3' away from the corner masts. In other words there will be three feet of halyard between the dipole and 10' metal mast, and there would be a dipole tail or remnant dangling down a few feet on each end. I would probably secure the ends of the dangling dipole with dacron rope so they don't flutter against the metal mast.

My question is whether 'tis better to let the ends dangle down or 'tis better to cut the lengths to my maximum useable space and use a tuner. As always, thoughts advice and opinions are most welcome.

 Frank W4FLN

i would go for the resonant length if possible ... changing directions at the ends has an almost unnoticible effect as long as they don't run back on themselves .... hanging ok

Let them dangle.

I have tried a dangling dipole for the same reasons you mention to get the electrical length.  What I found was the vertical components at the ends pick up a LOT of noise and the antenna was a noisy poor performer.  I would suggest you put up as much wire as you can as high as possible. Feed it dead center with balanced feeder, 450 ohm commercial or homemade as I do and use a good L tuner and a 4:1 or 1:1 voltage balun as close as possible to the tuner.  Dont get obsessed with resonance after all it is by definition when reactance is zero leaving only radiation resistance which could be anything.  The tuner will look after both.  In addition it does not need to be straight in the vertical or horizontal.  I have similar yard constraints and my dipole is fed at the roof line with each leg to a tree in the corners of my rear lot.  It is 90 ft per leg in a 90 degree horizontal V shape when looking up.  It tunes up and works very well on 160m and up even up on 6 meters and its quiet. At 56 ft per leg you should be good to go on 80m and above and Im betting it will work OK but not great on 160m.
Good luck, 73s  VE3ELQ

I had a similar situation many years ago. I tried this and it seemed to work well. The 20 foot lengths are connected together. I used a 5 inch piece of wood or phenolic as the separator. Used some ty-wraps to keep the separators from sliding on the wire. It wasn't elegant but it seemed to work. May have to do some minor pruning/adjusting but the tuner should take care of any issues.



        <   20 feet    > <         35 feet               >                 Same as the left side

 |     |---------------|--------------------------center----------------------------------------
5 in  |                     |
 |     |---------------|

I'll add a suggestion that you consider a "cage" dipole: at M.I.T., the W1MX station had a cage with about a 12" diameter, fed with open-wire line and a 4:1 balun. They had less than 2:1 SWR from 3.5 to 4 MHz, so I think a cage design will work well with shorter legs.

W1AC

I have the same issue and have had it for years.
I just make the antenna 10 feet short and make a coil of #12 wire, about 5 turns of an old coil form (pvc pipe would work).
1.2 to 1 swr at 3885.

A long time ago I used B+W coil stock over an antenna insulator, but the wire did not hold up long term.

Make it resonate, take your ends that would be dangling and wind them on 1 1/2" PVC similar to a screwdriver antenna without the coils touching each other. Zip tie, glue or machine a grove in the insulator and inset the wire. Kind of like a slinky if you pull it apart with a gap between coils. Tie the ends of the PVC to your poles. Electrically it will look like the same length. Experiment and have fun.
73
de k7iou

Franck C Jones antenna shown in an early Radio Handbook.  I made up the coils for an ham in Virginia.  One inch diameter ceramic end insulators wound with 12ga wire, I forgot how many turns.  I sent the coils to Dewit N4QNX and he said the antenna, with the coils, worked well.  Dewit moved to Ca but the antenna with the coils was left in Virginia.

I put my coils 3/4 the way out from center, I doubt it changes things much over a fill size antenna, its only 5 feet short a side and that gets the ends away from the trees.
I could hang the ends, but they would be IN the trees.
 

The coils I made were meant to be at the ends of the shorten dipole.  Dewit told me how many turns the coils had to be.  I never saw the plans for the antenna but it must be in one of the early Radio Handbooks. 

Coils at the ends has the least affect on the antenna's performance since the current at the ends is at a minimum.

The coils need to be bigger the more removed from the center I think.
#12 wire would not be a problem, even at the center.
I doubt it makes much difference at the far end where you put the coil.

Yep. Save winding coils for building transmitters.  ;)

That's important to keep in mind when compromising an antenna with loading coils or angles. The high current/low impedance portions of the antenna do most of the radiating although those points move as bands change.

Another idea.....

http://amfone.net/Amforum/index.php?topic=32911.0


klc

You're right, that's why the the coils I made for Dewit had many more turns than the ones you made for your antenna.

You're right except for coils at the end of a dipole.  End currents are always at a minimum regardless of the frequency.

This is all great stuff and food for thought. Thank you, gentlemen. :D

you can run back keeping 10 cm distance with spacers. It works for my dipole.

Don't over complicate things, just let the extra wire hang down. I have about six feet hanging down on either end of my 75 meter dipole. I run a KW match box with 600 ohm OWL. The thing plays great from 10 through 80.  Feed the thing with OWL line. Don't fool with a balun it is unnecessary and just something to smoke if you decide to run a legal limit.

73 John N8QPC

Unless you live in Pelham where the current is maximum at the ends.

Lot's of good opinions....for sure....   
I have two dipoles at my place.  East/West and North/South.
The lower dipole has a 30 foot apex and the ends droop down to about six feet from the ground.
I am resonant at 3900 KC and find the overall length of the dipole is 57' 3'' on each leg.
Keep in mind the capacitance effect of having the ends lower to the ground will require a shorter
antenna.
My East West is at 160' elevation and 61 feet seems to also be resonant at 3900 KC.   Both antennas
use a 5KW 1:1 balun.
You could also use wood poles as end supports and use standoffs coming down the poles to attach
the wire.   I have used electric fence insulators that are about 5 inches long.
Many use antenna tuners with good results but I live in a rural area with lot's of livestock and even
more electric fences.   Open ladder line and antenna tuners seem to pick up the electric fences better
than any Amateur signal....Hi.   
All my antennas are fed with LMR 400 and make use of a 1:1 balun at the feed point.
You could run loading coils as some suggested if you're not running high power.  If you intend to run high
power you might have problems unless the loading coils are really well made.
 

I do not think its a good idea to let the end of the antenna dangle down if it is IN the trees.
If I had to keep my antenna out of the trees, it would be much shorter then it is.

I am not sure what high power is.
I can run 1000 watts of carrier, and a coil that is 5 or 6 turns loose wound on a form, made with
insulated #12 wire is fine, has been fine for years.

The antennas are made with insulated wire, #10 and get very close to the trees yet do not change swr with snow, ice or rain. I never set anything on fire.
 

Letting the ends dangle will result in performance almost indistinguishable from a full-size inverted V.

But an inverted V with the apex at only 30 feet will not be a very efficient antenna on 75, especially at distances over 200 miles. Putting it up just 20 feet higher would make a dramatic improvement.

73,

Kevin, WB4AIO.

Aaaaand would make the 1/2 wavelength wire fit into the lot.

Pete,

I'm entirely unclear on how the ant that you suggests works?
If I understand it, what you've done is to make the end of the antenna "fat"??

As opposed to looping the end back under the main wire and not connecting the end back to
the main wire.

                      _-_-bear

I guess you can call it end-fat dipole. Sort of a take-off on a fan or bow-tie dipole. I came up with this back in the early 70's when I was living on a much smaller lot and didn't have enough room for a full size 75 meter inverted vee. I have no recollection of any background info that made me try this design. It might have been one of those mental revelations that pop into your head when you're sitting in the quiet place. Fed it with coax and a coiled balun. I seem to remember without the tuner the antenna had the best SWR around 3940. The total dimensional length was based on the space I had. The 20 foot length per leg was purely a "pull from the air" guesstimate. 

I've seen sites that mention something called "linear loading" for dipoles that do not quite have enough room.  I don't know how well the work but I thought I'd mention it.

Links and diagrams to follow...

http://www.m0pzt.com/?loadeddipole

http://w8mrc.com/docs/presentations/Shortening%20Your%20Dipole.pdf

http://www.af2cw.com/license/dipole.pdf

http://static.dxengineering.com/pdf/LL%20M-B%20Dipole.pdf

Linear loading can work pretty well, especially considering it usually requires minimal effort. Capacitive end loading (dangling ends are a form of this) and inductive loading with high-Q coils is more efficient. This is where many have gone wrong for years, they used low-Q coils.

I had a similar situation at my previous QTH. But rather than have the 15 foot ends dangle down I simply "bent" them horizontally instead in opposite directions of each other, sort of a zig and zag about 100 to 110 degrees from the main run. I had no SWR issues and it was resonant and seemed to perform quite well.  Center was at 45 feet though.