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Author Topic: Opinion rotatable or trap dipole  (Read 272 times)

K9SJB

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Opinion rotatable or trap dipole
« on: April 01, 2023, 08:19:11 AM »

Hi, I don’t have space for a tower and larger yagi so I use verticles and dipoles. My dipole has seen better days, built it probably 10 years ago. I’m wondering about a rotatable dipole or trapped dipole. Thoughts?
Thanks
73s K9SJB
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WA3SKN

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Re: Opinion rotatable or trap dipole
« Reply #1 on: April 01, 2023, 09:10:50 AM »

This depends upon height above ground in terms of wavelengths.
So what band(s) do you plan and how high can you get it?  I am assuming horizontal here, verticals have another set of "rules".

-Mike.
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K1VSK

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Re: Opinion rotatable or trap dipole
« Reply #2 on: April 01, 2023, 09:27:35 AM »

Hi, I don’t have space for a tower and larger yagi so I use verticles and dipoles. My dipole has seen better days, built it probably 10 years ago. I’m wondering about a rotatable dipole or trapped dipole. Thoughts?
Thanks
73s K9SJB
Rotatable dipoles and trapped dipoles are not mutually exclusive things.
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VR2AX

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Re: Opinion rotatable or trap dipole
« Reply #3 on: April 01, 2023, 10:29:12 AM »

Rigid horizontal versions do produce directivity at front/side, unlike inverted V ones, although centre height probably matters more than anything else with any half wave dipole. Height for height the simple dipole is only 5dB behind a typical 3 element mono band yagi, and will show a similar low angle radiation profile. A decent 20-10 trapped version like the Mosley (and some of its Asian versions  ;)) should not lose a significant part of the TX power as heat in the traps.
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AC7CW

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Re: Opinion rotatable or trap dipole
« Reply #4 on: April 01, 2023, 10:49:10 AM »

Every dipole I ever used has been noticeably directional. I've never had one that was rotatable but it seems like a good idea
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Novice 1958, 20WPM Extra now... (and get off my lawn)

WB6BYU

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Re: Opinion rotatable or trap dipole
« Reply #5 on: April 01, 2023, 12:11:52 PM »

My experience was the opposite:  I had a trap dipole
(driven element of a TA-33jr) that I could rotate
(Armstrong rotator).  I never noticed stations that
I could work when it was pointing one direction but
not in another direction.

For example, I was trying to work KL7 one Field Day
and aimed the dipole in that direction.  Nothing.
So I turned it back to the East Coast, and, when
the band opened, worked several KL7 stations without
needing to turn it.

So I just left the antenna roughly North/South and
didn't bother turning it.  Did I miss some possible
contacts? Probably.  Did it matter?  No.

Sure, I probably could have heard a difference if
I turned it while listening to a specific signal, and
the directivity would be greater with height and
longer distance contacts, and if you needed every
fraction of a dB to break through a pile-up.

But even running QRP, it just didn't make enough
difference in my day-to-day operation to justify
putting a rotator on it, other than the rope tied
to one end.


So it makes a difference what bands you are using,
what distances you want to cover, and your
operating preferences.  On the lower bands at
higher angles of radiation, there isn't much
directivity.

Rotating a wire dipole, of course, is more difficult,
but can be done with some clever rope work.

Trap dipoles can be built, although there may be
easier methods such as parasitic elements for
other bands, or multiple dipoles on a common
feedpoint, depending on how many bands you
want to cover.  Traps and loading to shorten an
antenna generally narrow the operating bandwidth,
but sometimes the shorter space makes it
more practical.

Mostly depends on the supports you have available
and the bands you want to cover.

NA6O

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Re: Opinion rotatable or trap dipole
« Reply #6 on: April 01, 2023, 01:04:38 PM »

If you have the space and the budget, a SteppIR UrbanBeam gives you 2 elements on 20-6m and is a dipole on 40 & 30. Has sort of a modern art look to it too…

Gary NA6O
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KL7CW

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Re: Opinion rotatable or trap dipole
« Reply #7 on: Today at 06:50:42 PM »

We really need more info to give you a good answer.  For now I will assume that you are talking about a 10,15,20 meter antenna. Also l will assume you want some good low angle radiation for Stateside and DX.  If this is correct and 20 meters is important, try hard to get it up as high as possible.  It will work at 25 or 30 feet, but much better if you can get it up say 40 feet or more.  Lower heights required for 10 and 15 meters.  I mostly agree with WB6BTU since re: the radiation pattern of a dipole.  However I have found in my experiments on 30 meters a dipole does have very significant nulls DIRECTLY off the ends for both the path (3000 miles) to either WWV or WWVH from my Alaska QTH, and local tests with transmitters less than 5 miles away on 10 MHz.  Now the pattern, both the model and experimental, agree with each other VERY closely.  The pattern roughly broadside to a dipole, shortened or full size, is very broad, so over much of the 360 degrees, with atmospheric fades, etc. probably no need to rotate.  However, off the ends the nulls, both the models and measured, averaged at least 15 dB deep if nearly directly off the ends.  Sorry I did these tests decades ago, so lets say over something like + or - 5 degrees from directly off the ends, and occasionally if I carefully adjusted the rotator the nulls were MANY S units deep.  Often, I have good propagation to both WWV (great circle E), and WWVH (great circle S) from my location.  Just turning my dipole 90 degrees often lets me clearly hear only WWV when aimed E, and only WWVH when aimed S. If your dipole is a wire and horizontal, and at the same height as the rotary dipole, then not much difference in many directions, BUT if the wire dipole is an inverted V or even something like 10 feet lower, than the rotary one may be worth the cost and effort.  When I work a whole region of the world, I do not bother adjusting the 30 or 40 meter dipole for say different countries in Europe, or most of the lower 48 states, but for sure it is down a few S units directly off the ends.  I do not have problems with trap antennas, in many situations the loss is small and the versatility is worth it as long as the traps are designed for your power and you can live with reduced bandwidth.   Rick  KL7CW





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