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Author Topic: dipole attached to the top of a verticle antenna - can this be done  (Read 2315 times)
KF7NUA
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Posts: 153




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« on: October 09, 2012, 10:08:04 AM »

I didn't want to ask in a post that belonged to some one else so I decided to ask here. I have seen this type of reply a few times and I am not sure if it is possible to do or if there are limitations. Is this possible to do without interactions? The replies are similar to this....

"I have a verticle antenna and I have a dipole attached to the top of it"

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W9GB
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Posts: 2599




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« Reply #1 on: October 09, 2012, 10:59:13 AM »

Nicolo -

START with the TWO very basic antennas from the beginnng of Radio, a century ago.

The HERTZ (1/2-wavelength Dipole) and the MARCONI (1/4-wavelength Vertical) antenna.

German physicist Heinrich Hertz invented/discovered the classic dipole antenna around 1886 in his pioneering experiment with Radio waves, as predicted by James C. Maxwell.
http://en.wikipedia.org/wiki/Dipole_antenna

In 1895 Italian radio pioneer Guglielmo Marconi, while testing early radio apparatus in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with long wire "aerials".
The term "Marconi antenna" usually refers to a two part antenna consisting of a vertical portion and a "reflective" or "ground" portion. When constructed properly, it is very similar to a vertically oriented dipole, in that one element is "up" and the other "down".
The reflective portion is not always a physical element, but often either natural earth ground (where the soil conductivity is sufficient) or ground "radials" - a set of wires along or just beneath the ground that act as the reflective portion.
A Marconi antenna is an omni-directional (same transmit/receive in all directions) antenna that has good long distance characteristics on HF (high frequency) and MW (medium wave, or AM) frequencies.
A Marconi is typically built with a 1/4 wavelength vertical element, and similar length radial(s).
For example, the full wavelength for 7 MHz is about 133 feet (~ 40 meters).
A 1/4 wavelength vertical element (and each radial) would therefore be about 33.5 feet.

Most AM broadcast stations use some variation of a Marconi antenna.
Since an AM station at 1050 on an AM dial is equal to 1.050 MHz and therefore a wavelength of over 990 feet, a 1/4-wavelength vertical element would be almost 223 feet for that station!

This is why AM broadcast stations use a structural tower, insulated from the ground, and an extensive radial field for broadcasting.
==
Counterpoise?  On the Use and Abuse of a Word, December 2006.
L. B. Cebik, W4RNL (SK)
http://www.antennex.com/shack/Dec06/cps.html
==
Radio Antennas 101
http://en.wikipedia.org/wiki/Antenna_(radio)
==
The word "Antenna" came from Marconi and the Italian language.

In Marconi's early radio experiments he used a 2.5 meter vertical pole, with a wire attached to the top running down to the transmitter, as a radiating and receiving aerial element.
In Italian (language) a tent pole is known as l'antenna centrale, and the pole with the wire was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals.

Because of his prominence, Marconi's use of the word antenna (Italian for pole) spread among wireless researchers, and later to the general public.
Marconi's father was Italian, and both men spoke Italian fluently.
« Last Edit: October 09, 2012, 11:05:24 AM by W9GB » Logged
K0ZN
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Posts: 1533




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« Reply #2 on: October 09, 2012, 05:50:54 PM »

"Can" you do that?? .....Yes....   Is it a good engineering or antenna design practice??  No.

The issue is how close the two antennas are to each other.  If you could run a pretty decent length of NON-conductor, such as rope, from the top of the vertical to get the dipole well away from the vertical, the situation would likely be much better and maybe fine.  If I had an 80 M vertical made from a tower structure, I would not be concerned to use it as a support for one end of a dipole if the near end of the dipole was probably at least 35 ft or so from the vertical. I absolutely would not
want the end of the dipole right up near the vertical. Obviously, available space and real estate significantly impact we can as hams can do..... it is a hobby
and we HAVE to "make do" in many cases; that is just reality.

Generally, a vertical, particularly a 1/4 wave vertical is not too happy with other conductors in the close proximity of the antenna.
Interaction can cause de-tuning, change the feedpoint impedance and possibly distort the radiation pattern.   Worst case, by far,
would be a dipole cut for the same band as the vertical. The feedline from the dipole dropping vertically to the ground, etc. very
likely would couple to some extent and with some effect on the vertical. Bottomline: the effect is hard to define as it is distance related.

There are a lot of things that happen in ham radio installations that "work"..... doesn't mean because it "works" it is a
good design or good engineering. LOTS of things and various forms of "ham haywire" will radiate to a useable extent and one can and
does make contacts, but the station most likely would get out better, have stronger signals and less "issues" if the antenna and feed system was a
GOOD engineering practice.

Time spent doing a little studying in the ARRL Antenna Book or other good antenna book will educate and help explain the impact of other conductors
in the near field of an antenna in more detail. (Keep in mind, sometimes another conductor is WANTED.....such as a reflector element, etc.....
and some times a conductor is not wanted, such as a guy wire in some cases.)

73,  K0ZN

« Last Edit: October 09, 2012, 05:55:36 PM by K0ZN » Logged
WB6BYU
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Posts: 13026




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« Reply #3 on: October 09, 2012, 08:16:03 PM »

Yes, it can be done, depending on exactly what you are thinking of.

Using the vertical as a support to tie off one end of a dipole is OK, as long as the rope
between them is low enough to minimize interaction, except that many verticals aren't
stiff enough to withstand much sideways strain at the top.


If you want to use a vertical as the primary support for a dipole, the typical approach
would be to run the dipole feedline down the INSIDE of the vertical, or even use the
dipole feedline as the radiator for the vertcial.  And the dipole likely would become
part of the vertical, depending on how it is fed.

For example, you could make a rotatable dipole for 20m using aluminum tubing fed with
twinlead, then for 80m you would short the two ends of the twinlead together and
feed them against ground to make a quarter wave vertical with the 20m dipole providing
some of the top loading.

Occasionally you might come across a design for the "Mul-Tee" antenna, which was a
folded dipole for one band with a quarter wave matching section of open wire line, that
together formed a vertical on another band.  (Although it doesn't really work like that
in practice...)


It isn't uncommon to see someone use a tower with a beam on top as a vertical radiator
for 80m or 160m, while still being able to use the beam in the normal manner on the
higher bands.  But the beam dimensions affect the resonant frequency of the tower,
and the feedlines and rotator cables have to be suitably decoupled or they will have
a lot of RF flowing on them.  Typically such towers use some form of shunt feeding
so the base can remain solidly grounded for lightning protection.

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KF7NUA
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Posts: 153




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« Reply #4 on: October 10, 2012, 06:21:19 AM »

thank you all for the responses.
First off this is not something I was going to try but it was something I had seen written and thought it could not work effectively.
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W8JI
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Posts: 9304


WWW

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« Reply #5 on: October 10, 2012, 07:08:33 AM »

The only accurate answer to this, like many other antenna problems or questions is, "It depends".

thank you all for the responses.
First off this is not something I was going to try but it was something I had seen written and thought it could not work effectively.


It can work VERY effectively, if properly implemented. As a matter of fact a dipole or two can actually augment the vertical's performance.

Of course if improperly done, as with many things in antennas, it can hurt performance.

Most of my life I have shared verticals with supporting other antennas, and it never has been deleterious to the vertical or the other antenna. Hundreds of other Hams also do the same. Sharing simply requires reasonable planning, so we cannot willy-nilly do anything we want, but with proper planning it can even make things work BETTER.

73 Tom
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LA9XSA
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Posts: 376




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« Reply #6 on: October 10, 2012, 03:18:53 PM »

Yeah as far as I understand, the typical thing is to have - say - a 20 meter beam, or perhaps a trapped beam for 20 to 15 meters on a tower (substitute for a 20 meter dipole if you want less directivity), and then load up the tower itself for - say - 160 meters vertical. Rather than isolating the tower from ground (which would be hard to do with that feedline going to the beam), what you do is keep the tower grounded but move the feed up and down the tower, you tap it, combined with adding capacitance or inductance, until you get a close to 50 ohm + 0j  impedance on your chosen frequency.

Note here that one usually does this with bands that are far apart. Putting a dipole on top of a vertical of the same band, or a nearby band, would lead to the problems that K0ZN's post is about

Check out Tom's webiste W8JI.com - there's plenty of information there. For example here: http://www.w8ji.com/omega_and_gama_matching.htm or in the ARRL Antenna Book.

PS: W9GB, are you sure you responded to the right thread?
« Last Edit: October 10, 2012, 03:22:39 PM by LA9XSA » Logged
KF7NUA
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Posts: 153




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« Reply #7 on: October 10, 2012, 03:59:34 PM »

I think the problem I had when reading this type of phrase is that when I see the word Verticle I think R7, 6BTV, or S9 and the like, not a tower!
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K3VAT
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Posts: 701




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« Reply #8 on: October 10, 2012, 05:06:19 PM »

... As a matter of fact a dipole or two can actually augment the vertical's performance.
...  73 Tom  

Tom,

Could you please give us some examples to your comment above "dipole augmentation of a vertical"?

For instance, I have a 40M tree vertical (several #14 insulated wires & using standoff insulators) at 33' with fair radial field (eighteen ~50 footers in a circle).  I was contemplating in constructing a 30M fixed aluminum dipole at some convenient height above it.  For this example, is there a way for the 30M dipole to augment the 40M vertical? 80M perhaps? thanks, 73, Rich, K3VAT


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W8JI
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« Reply #9 on: October 10, 2012, 05:35:05 PM »

The bands do not need to be far apart.

For example, a 20, 15, and 10 meter dipole at the top of a 40 foot mast could be used to guy the mast, and the mast could be used as a vertical on 160 and 80 with improved results and 40 with similar or slightly improved results.

When I had limited space, I had a 40 foot push up insulated mast with a 20 through 10 inverted vee trap dipole at the top. I fed the mast on 160-40 as a vertical, and the dipoles on higher bands.

It was better on all bands than it would have been as a vertical without the dipole.

It isn't easily possible to do this with a trap vertical, but it could easily be done as a stub decoupled vertical similar to a Hy-tower. Antennas manufacturers don't do things like this because they are generally not run by antenna engineers.

73 Tom
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WB6BYU
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Posts: 13026




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« Reply #10 on: October 10, 2012, 06:44:38 PM »

Quote from: KF7NUA
I think the problem I had when reading this type of phrase is that when I see the word Verticle I think R7, 6BTV, or S9 and the like, not a tower!


It depends whether you are talking about a verticle or a vertical I suppose...


A tower, or any other conductor, can be used as a vertical antenna.  So can a piece
of wire hung from a tree branch.


So let's take the example of the 40m vertical made from two parallel wires, and a
proposed 30m dipole.  I'd use the two vertical wires as a feedline for the 30m dipole,
as well as for the 40m vertical.  That gets the maximum current up higher in the
air and raises the feedpoint impedance.   (It also makes an even big improvement
if you are trying to use the same antenna on 80m.)  The dipole acts as a top hat
for the vertical when the wires are fed together against ground, and as a dipole
when the wires are fed differentially.

I'd want to model it before recommending a specific set of dimensions, but you
get the idea.
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WB6BYU
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« Reply #11 on: October 10, 2012, 08:33:20 PM »

EZNEC suggests the addition of the 30m dipole to the top of the 40m antenna
makes about a 1.5dB improvement in maximum signal strength with a somewhat
lower angle of radiation.
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ZL3OF
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Posts: 20




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« Reply #12 on: October 11, 2012, 02:57:38 AM »

Many years ago at another QTH I had a tower with a tribander, 6m and 2m beams on it and a 80/40 trap dipole strung off it. I installed some radials and used the tower as a vertical on 160/80/40. Great dx antenna, worked Sweden with 100 watts SSB on 80 metres from ZL. The dipole was used for local QSOs.

However I found that when using the vertical tower on 80 or 40 metres, shorting out the dipole feedline at the shack end would increase the reported signal strength when transmitting by a minimum of 6db. The dipole was working like a giant absorption wavemeter. I guess shorting out the feedline detuned the dipole.
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W8JI
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« Reply #13 on: October 11, 2012, 07:56:53 PM »

==
Counterpoise?  On the Use and Abuse of a Word, December 2006.
L. B. Cebik, W4RNL (SK)
http://www.antennex.com/shack/Dec06/cps.html

Cebik was incorrect.

Counterpoise:

http://www.w8ji.com/counterpoise_systems.htm

73 Tom
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