20m dipole, best construction?

[VE3GZB]

I looked at using some PVC pipe, suspending it horizontally at the top of the TV mast. But even a 10 foot length of the stuff was too flexible.

Can anyone suggest any examples or designs for a 20m mast-top style of dipole which can be home constructed?

Thanks and 73s
geo VE3GZB

[W4AMV]

Take a look at the MOXON beam construction techniques. The use of a central hub with spreaders for a 2-element yagi is discussed by many folks. In your case, just ONE spreader (i.e. 2-pieces of fir strips supported by the hub) would do your dipole. I think there is discussion on this forum for the MOXON, otherwise check the web. You can build the hub out of a piece of nylon cutting board from you wife's kitchen, before she throws it out, and the spreader can be constructed from light pieces of fir suitable varnish or weather proofing should be added. If that is to long, then consider adding inductance at the feed point in series with the elements. The L could be formed from several turns of 1/4 inch copper refrigerator tubing. This is in the spirit of the same provided by the ARRL handbook back in 73' for a 15 meter rotary dipole. 

Alan

[VE3GZB]

The Moxon looks pretty nice!! I might give it a try!

I wonder if anyone has tried this G3TPW cobweb antenna that I found here:

http://www.g3tpw.co.uk/

Any thoughts? In some ways it almost seems similar to the moxon design?

Thanks and 73s, geo VE3GZB

[W4AMV]

Thanks for the heads up on this antenna. The description implies it is a full dipole in length although apperance is that of a loop. The construction technique is certainly MOXON in style. It would be nice to place it into NEC and validate the gain, pattern and take off angle.

[VE3GZB]

Gee, I'm trying to wrap my mind around the Moxon antenna and the Cobwebb antenna, doesn't seem like they would actually work to me given my understanding of a dipole....it seems that the folded portion would cancel any radiation from the opposing parallel portion.

I'm not sure I understand the real need for a balun (I despise the notion of having to use anything with a ferrous magnetic core, it just begs for trouble).

Has anyone here built a Moxon antenna for 20m and did it with just a straight transmission line feed, no balun? I'd appreciate hearing your experience.

73s geo VE3GZB

[KB2WIG]

G,

Lots 'o info here.....

http://www.cebik.com/

and it's fre.... Ya just gotta sign up fer it.


klc

[K1JJ]

Geo,

Moxons work very well and model out with even better front-back ratios than 2el Yagis.  They see up to 25db f-b vs: 15db f-b of a 2el Yagi.

They may be more difficult to support the els vs: a simple 2el Yagi however.

Another alternative for you, if you do not want the complications on a 2el Yagi or Moxon, is to build a single rotatable dipole.  I don't like traps, but even the driven element dipole from an old TA-33 tribander (or any tribander) would get you by for the higher bands 10-20M.  

For the single band dipole, you could mount the alum elements on a flat piece of aluminum stock and use a "T" match. This requires no element to mast isolation - the eles are grounded at the mastplate and fed thru the T. Look  up "T match"  on the web.

BTW, for a 1:1 RF balun, just use 6 turns of coax from the feedpoint coiled up on a short 3" PVC pipe. No loss to speak of.

Hope these ideas will get the gears turning.  
T

[Steve - K4HX]

The Moxon elements are no longer than the elements of a Yagi, so I don't see why they would be harder to support. A simple X support is all that is needed. Make the elements of wire and the thing si rather lightweight.

[VE3GZB]

My biggest concern is having horizontal elements which are not rigid enough or will flex too much, droop too much just under their own weight.

My preference is for the dipole - it's a proven bit of technology - but if I were to construct any antenna and stick it up 35 feet in the air, what materials should I use to construct the horizontal elements? My hope of using PVC pipe is up in smoke because even a mere 10 foot length sags like a wet noodle just under it's own weight.

In keeping with the neighbours and keeping my wife happy, it should be as unobtrusive as possible and look not much different from a Yagi which Television viewers have been using since the 60s.

[VE3GZB]

The idea occurred to me, if I buy two of these 10m whips:

http://www.durhamradio.com/jetstream-jtmhf10-mobile-hf-amateur-whip-antenna-10-meters-38-24-thread.html

And if I buy two mounting bases then if I make a mounting thing and poise the whips horizontally opposing each other, pointing away, do you think this would be good as a 20m dipole? Has anyone ever tried this before?

What is the maximum power rating for these whips and bases? And would these whips sag much?

73s geo VE3GZB

[K1JJ]

The Moxon elements are no longer than the elements of a Yagi, so I don't see why they would be harder to support. A simple X support is all that is needed. Make the elements of wire and the thing si rather lightweight.

What I mean by harder to support is the added complexity of using both solid supports and wire for the Moxon.  The Yagi has just two straight elements.  Even so, Geo may prefer something more "neighbor friendly" that will look like a TV antenna.  

Actually, a short log periodic made for 10-20M would look more like a TV antenna once it's up in the air...

T

[K1JJ]

The idea occurred to me, if I buy two of these 10m whips:

http://www.durhamradio.com/jetstream-jtmhf10-mobile-hf-amateur-whip-antenna-10-meters-38-24-thread.html

And if I buy two mounting bases then if I make a mounting thing and poise the whips horizontally opposing each other, pointing away, do you think this would be good as a 20m dipole? Has anyone ever tried this before?

What is the maximum power rating for these whips and bases? And would these whips sag much?

73s geo VE3GZB

Yes, whips are a great idea for easily supported rotary dipoles.   Though, I would go with the standard 9' stainless steel CB whip, like the ones sold at Radio shack for about $15 years ago. I once used four for a DFing Adcock antenna. Worked FB as a pair of phased dipoles.

The whip shown in your link is rated at 300w pep and has a coil, etc. This is NG.   Go with a solid whip and use a pair of small 1/8" tubing copper coils on the input to tune it to resonance as a 20M dipole. Or just trim the whip ends if they are already long enough for 14.2 mhz as a dipole.  Wind up 6 turns of coax as the 1:1 coaxial balun.

T

[VE3GZB]

How about this whip?

http://www.durhamradio.com/phf10-10m-hf-mobile-whip-ontario-canada.html

73s geo

[K1JJ]

That whip may be too short unless you want to add coils on the input.

Find a pair of 9' stainless steel CB whips. They are made for 27 mhz, thus longer than needed for 20M without using coils.   They are stronger than any other hybrid whip with less wind loading too.

Here's what you want, but look for it cheaper priced:

http://cgi.ebay.com/New-102-Inch-CB-Ham-Radio-Antenna-Stain-Steel-Whip-/120712485522?pt=LH_DefaultDomain_0&hash=item1c1b065a92

T

[W4AMV]

oh boy what FUN.
  Just for the grins, consider the shortest cheapest stuff out there, electrical conduit, available in 10 ft lengths. Less than 2$ here at the hardware store. Then add a copper center loaded coil, need 5 uH. At 14 MHz, (CW), if this dipole is up 20-40 feet, port Z is about  20-30 ohm range and take off angle is in the vicinity of 30 degrees and the gain is 6 or so dBi. Placed in EZNEC just for fun. Need to play with inductance equation as to what diameter copper coil I need. Any flys in the ointment! 

[K1JJ]

Yep, that's a good idea using the conduit.

Another idea is to go to the metal recyling scrap yards and pick out some standard aluminum tubing. They usually set it aside for retail sales. I was able to build a whole Yagi farm using $1/pound aluminum tubing and pipe from these scrap yards. Most of it was new.  It looked like they had a 20 year "picking" stashed away. I cleaned them out good..

Some of these aluminum tubes will telescope together, or you might need to make sleeves.

Last resort is to check out Texas Towers for their aluminum tubing. This stuff telescopes and makes dandy Yagi material. Geo, for the money you are willing to pay for these whips, the Texas Tower tubes are a good deal too.  Get 6' sections that are UPS ship-able and slide them together.

Check it out:
http://www.texastowers.com/aluminum_.htm

T

[Pete, WA2CWA]

You could do this from Lakeview Company, http://www.hamstick.com/products.html
Get one of these:

#901 Rotatable dipole mount

Get two 20 meter #9120 "Hamstick" mobile antennas:

Connect coax, mount assembly to the pipe...done.

[W4AMV]

Pete, yes, that will do it. Just a little pricey but in terms of time investement, no contest.

[KB2WIG]

An old ARrrrL antenner handbook had a armstrong dipole made out of  lectrical conduit and a loading coil......       cheep to build.

klc

[WB6NVH]

Why use a dipole?  An inverted V is easier to construct, although for 20 Meters there are some easy to make designs which give some directivity, as mentioned above.

[KA2DZT]

The Moxon elements are no longer than the elements of a Yagi, so I don't see why they would be harder to support. A simple X support is all that is needed. Make the elements of wire and the thing si rather lightweight.

What I mean by harder to support is the added complexity of using both solid supports and wire for the Moxon.  The Yagi has just two straight elements.  Even so, Geo may prefer something more "neighbor friendly" that will look like a TV antenna.  

Actually, a short log periodic made for 10-20M would look more like a TV antenna once it's up in the air...

T

Had many customers ask if they would see the TV antenna once I installed it on their roof.

I answered, only if they looked up.

Another side note;  the higher you get them up,  the smaller they look from the ground.

Fred

[ve6pg]

..geo, as mentioned, 2 20m hamsticks will work...one on the centre connection, the other on the braid of the coax...the antenna will be about 16 feet wide...all those hamsticks, (like the ones at durham), are all the same length, about 8 feet each...the difference is how much wire is wound on the antenna...i have some here, and the 80m one is the same physical length as the 10m one....they come up all the time, at hamfests, for about 10 dollars each...check the ontarioswapshop, or do a google for KWARC....kitchener/waterloo web page...they have a good swap page there..i was through grand valley today, down to mississauga to pick up a beam..was thru your town twice, in about 4 hours....
..tim..

..sk..