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Author Topic: Rotatable dipole for 20 meters - U or T ends?  (Read 1023 times)

XW0LP

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Rotatable dipole for 20 meters - U or T ends?
« on: September 28, 2019, 05:39:50 AM »

In my quest to find a suitable monoband antenna for my handkerchief-sized lot, I have considered EFHW and inverted V antennas (in another post in this forum).

But wouldn't it be great if I could erect a dipole antenna that I can rotate?

Here's my idea:

My house is built of wood, with clay roof tiles. The root is a typical pitched roof.  Luckily for me, it is very easy to slide the small clay tiles to one side and push up a vertical antenna support, (that exit hole does have to be sealed to a degree to stop rain ingress, but still allow the support mast to be turned by hand).

The mast will be a 1.5 inch blue water pipe, extending about 2-3 meters above the roof line. No guy wire is needed because the length of pipe is supported as it passes through the tile hole and is also supported within my attic. The antenna will be a wire dipole antenna that is taped to  two horizontal cross-pieces of 1 inch plastic pipe, each about 3 meters long, and joined to the support mast, with the coaxial feed cable running up the mast pipe.

Each dipole element needs to be about 5 meters long.  I know that I can bend the dipole at about 60% of its overall length, without affecting the radiation pattern too badly, and that 60% location will be at about 3 meters.  Then I use another length of 3/4 inch plastic pipe that is 2 meters long, secured to the end of the 3 meter pipe and hanging down, thus obtaining my overall 5 meters.

I've tested the mechanical strength of this idea and it all looks fine, because this antenna is mechanically symmetrical and lightweight, and my location is sheltered from strong winds.

The other end of my support mast actually passes through the ceiling of the room below (my shack), and the end rests on the room floor.  I can simply rotate the mast and antenna by hand from the comfort of my room!

I can build such an antenna...  but I want to know if it would be better mechanically and electrically if the 2 meter end sections were connected as a T piece at the end of the 3 meter length, rather than hanging down 2 meters.  ie - the end piece would project up 1 meter and project down 1 meter.

In that T piece design, how would I electronically connect the 2 meters of dipole wire to the end of the 3 meter length of wire?  Would I still have to use the same length of wire in order to achieve resonance on 20 meters?  Are there any advantages or disadvantages of using a T end section or a end section that only projects downwards?  Thanks as always for advice and comments :)
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KL7CW

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #1 on: October 01, 2019, 08:06:35 AM »

Mechanically you may have some issues with plastic pipe for your boom.  Some of this type of pipe will be fine for awhile, but then tend to sag, especially in hot sunny weather.  If we place the wire inside the boom, the dielectric effect of the pipe will probably make the dipole electrically act like it is at least a few percent longer.  If you use insulated wire this may even make the dipole act an additional few percent longer.  If the pipe is non conductive, results should be fine and your end sections will be slightly shorter.  Now, T end sections will have only a slight electrical advantage, however the overall length of the T will need to be somewhat longer than the simple "hanging" end sections.  Possibly something like 3 meters instead of 2 meters.  It will weigh more also.  If possible the lower end of the hanging section should be well clear of the roof, hopefully something like at least a meter, but less is OK if necessary.  Your antenna should work fine, but the Z at the feed point will probably be less than 50  ohms, possibly as low as 30 or 40 ohms.  You may want to consider just hanging a vertical wire at the end of each element.  It will be much lighter, and even something like a 100 g weight should keep it relatively stable.  I do not know if the ends moving in the wind will have any significant effect on the antenna.  Please consider other construction options such as aluminum, fiberglass, etc.  If you use T sections on the end, why not make them horizontal.  That way the ends of the dipole will be well clear of the roof.  To save weight you could even use something like a fiberglass rod like bicycle riders use for visibility with a flag on top.  Just tape your wire to the rod.  Just drill a hole near the end of your boom so the rod will tightly fit.  I have used these rods and they are strong, however in sunny climates they should be protected with paint or they will deteriorate in a few years.       Good Luck   Rick  KL7CW    Palmer, Alaska
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KH6AQ

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #2 on: October 01, 2019, 12:37:58 PM »

Mechanical
The ends that project downward only (L-configuration) will tend to twist the horizontal element in a wind. With the end piece going up and down (T-configuration) there will be little wind torque. Either might be fine in your wind sheltered environment.

Electrical
An EZNEC model of the 3 x 2 meter L-configurations is resonant close at 14.2 MHz. The 3 x 1.2 meter T-configuration is also resonant close to 14.2 MHz. Your lengths may be shorter due to PVC dielectric loading.

I'd like to hear how this interesting antenna works for you.
« Last Edit: October 01, 2019, 12:41:15 PM by KH6AQ »
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XW0LP

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #3 on: October 01, 2019, 04:12:34 PM »

Quote
You may want to consider just hanging a vertical wire at the end of each element.

Exactly.  This is something that I also thought about.  It sounds quite a good idea :)
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KL7CW

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #4 on: October 02, 2019, 04:25:36 PM »

I have built antennas with the T configuration (the T horizontal) and the performance should be less than 0.5 dB down from a full size dipole.  The pattern will be slightly broader than a regular dipole which may be a slight advantage with your "Armstrong" method of rotation.  The Z will be somewhat lower, but if this is a problem it is easy to get to 50 ohms with something like around a 2:1 balun, or even short transmission line transformer made of coax, or a simple L network, or a shunt inductor.  If the antenna is going to be somewhat permanent, then a balun at the feed point may help, but will not hurt.  If you like the idea of small (horizontal) fiberglass rods at the ends of the boom, but they are a bit too short do not worry.  Just use a small inductor in series with each element at the feed point.  Normally we avoid this solution, however if you only need to move the resonance down a few hundred kHz on 14 MHz the additional loss will be nearly zero (I have used this solution for years and modeled it many times).  This inductor will probably be only a few turns of wire perhaps something like 3 cm in diameter.  If a coil is needed just do something like 6 turns, the resonance will probably be too low, then remove a turn or two at a time.  Just wind the coil in the air, and final adjustments can be made by compressing (increases inductance) or expanding the coil which decreases the inductance.  Much faster than lots of calculations and modeling !!  The inductor idea only works efficiently for small changes in resonance, if you need a large coil, losses, SWR BW, etc. quickly become significant.   KL7CW
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WB6BYU

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #5 on: October 04, 2019, 04:28:01 PM »

Quote
You may want to consider just hanging a vertical wire at the end of each element.

Exactly.  This is something that I also thought about.  It sounds quite a good idea :)



Here is W4RNL's article on using an "inverted U" elements for a 2-element beam.
He designed the horizontal elements for 10m, and hung extensions (made from aluminum
electric fence wire) from the ends for the other bands.  His comment was that it worked OK
down to 17m, but not particularly well on 20m.  (A longer boom would help there, too.)

If you designed the base elements for 15m then they should work on 17m and 20m, and you
could telescope them if you also wanted 10m or 12m.

W4FID

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #6 on: October 15, 2019, 04:37:40 AM »

Two concerns.

Will rain "slide" down the vertical tube and into the house? If the hole thru the roof is loose enough so you can rotate the mast doesn't that mean there is a little space and surface effect moisture will allow some to get in? Also condensation with temp/wx changes. Will there be moisture inside the mast that will come down the inside of the mast?

Have you considered a simple hamstick dipole? Yes, they are shortened and have a tighter SWR range. But at 20 meters they are not too bad. Since they are already made and wx resistant -- made for the wind and rain in a mobile on the highway. Mounting brackets are readily available. The narrow bandwidth also means high Q which means fairly efficient at resonance. Pick a freq you use most often. Use a tuner at the rig to broaden out your range some and you'll cover most of 20 meters. I would think a hamstick dipole would have less cross section and less length that what you fabricate and be more mechanically solid.
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K0UA

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #7 on: October 15, 2019, 07:32:58 PM »

A simple cone shaped rubber "boot" will solve the rain down the pipe problem.  Affixed and sealed tightly to the mast at the top, and flaring out at the bottom to cover his sleeve the mast is stuck up through.  No rain can get into the house as water doesn't run uphill. As long as the boot covers the sleeve the mast is stuck thru and the sleeve is waterproofed sealed around the house roof, it will work out fine.
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73  James K0UA

XW0LP

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RE: Rotatable dipole for 20 meters - U or T ends?
« Reply #8 on: October 15, 2019, 08:22:32 PM »

Well, having got a local guy to build a bamboo ladder, I got access to the attic space and decided that installing a rotatable dipole wasn't going to be easy...

Considering all options and previous on-the-air experiences, I decided to install an EFHW vertical antenna for 20 metres.  The wire is taped to a fibreglass, telescopic mast that is pushed up 10 metres through a hole in the roof where I slid a tile to one side.  The matching transformer sits inside the attic, just under the tiles and therefore protected from rain.

A few hours after I installed this mast, it rained like crazy and floods of water came in, before I had sealed round the mast!  I was in my attic in the middle of the night with buckets to catch the water.  The next day, I sealed the hole around the mast with silicone and expanded foam.  When the heavy rain came down a few days later, everything was 99.5% dry inside the attic :)

For my current QTH, this is the best that I can realistically achieve for an antenna.  But it works pretty well, with confirmed QSOs into east and west coast NA, and a 'hotspot' path into coastal Brazil, where I've managed 4 QSOs in a few days (around 0130 am local time).



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KK4OBI

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Re: Rotatable dipole for 20 meters - U or T ends?
« Reply #9 on: November 18, 2019, 07:40:29 PM »

You can also bend the ends of your dipole down and then fold the ends back.  Tuning is by adjusting the length of the folded ends to a get a good match for 50 Ohm coax.  Performance is virtually the same as a straight dipole.

Here are the details and an antenna model to play with:
www.qsl.net/kk4obi/Center-fed%20Folded-end%20Horizontal.html
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