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Author Topic: Dipole choices - I thought I new what I wanted but.....  (Read 5396 times)
KF7NUA
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Posts: 153




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« on: September 17, 2012, 06:20:41 PM »

After a long learning experience on traps and loaded coils I have a question for the experts.
My needs are 20m and 40m but I would like to see what 75m and 80m is like.
I was just about to make separate 20m and 40m dipoles and then add additional loading coils on the 40m to also gain 75m or 80m too. I know that 75 or 80 will be very narrow bandwidth.
Question is:
Skip making the above 20m, 40m and added coils for 75m or 80m and just put up a 102ft G5RV.
My only problem is I will need to drop down the last 10ft or so on each end because I do not have the room.
Is this a better choice?
Will the G5 have better bandwidth?
I also think the G5 would be much cheaper to do.

thanks Nick
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WB2WIK
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Posts: 20611




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« Reply #1 on: September 17, 2012, 06:24:39 PM »

Droppin the ends down is fine, as long as they remain high enough at the ends to be out of reach of anyone.  Probably 8-9' above ground would guarantee that unless you have NBA Centers roaming your yard.

You'll need a tuner for the G5RV on 80m, but it's actually a pretty good antenna on 75/80.  It just doesn't match well across the band, and almost always needs a tuner.  On 40-20-12 meters it usually doesn't need a tuner.  On other bands, even with a tuner it's not much of an antenna as the feedpoint mismatch is terrible.
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W9GB
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Posts: 2626




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« Reply #2 on: September 17, 2012, 06:33:19 PM »

Quote from: WB2WIK
Droppin the ends down is fine, as long as they remain high enough at the ends to be out of reach of anyone.  Probably 8-9' above ground would guarantee that unless you have NBA Centers roaming your yard.
Nick -

You may have a question, WHY DOES THAT MATTER?.
The ends of antennas are most often the High-Voltage points !!!  You use insulators for a reason.
« Last Edit: September 17, 2012, 06:35:27 PM by W9GB » Logged
KF7NUA
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Posts: 153




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« Reply #3 on: September 17, 2012, 06:43:48 PM »

Quote from: WB2WIK
Droppin the ends down is fine, as long as they remain high enough at the ends to be out of reach of anyone.  Probably 8-9' above ground would guarantee that unless you have NBA Centers roaming your yard.
Nick -

You may have a question, WHY DOES THAT MATTER?.
The ends of antennas are most often the High-Voltage points !!!  You use insulators for a reason.

Understood. Maybe i might be able to extend them on an angle. I have no children but this is the desert so wild animals could be a problem.   
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WB6BYU
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Posts: 13336




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« Reply #4 on: September 17, 2012, 06:54:26 PM »

My previous suggestion was to add the 80m loading coils and extension wires to
the 20m wires rather than the 40m wires.  You can then get all three bands in
the same space as a 40m dipole.

Here is an example (though you don't need to follow all the details):

http://www.hamuniverse.com/ae5jufielddayantenna.html
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AA5WG
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Posts: 498




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« Reply #5 on: September 18, 2012, 06:39:56 AM »

Nick:

Home brew a link antenna coupler, some ladder line and a horizontal antenna for the lowest band in use.  This
will give you all band use on all bands plus the ability to series tune for a low impedance antenna system and parallel tune for high impedance antenna system.  

The ability and flexibility to tune either a low or high impedance antenna systems is dependent upon your antenna system length, i.e. one half the flat top length plus ladder line length measured in multiple 1/4 wavelengths.  Changing bands can change your impedance characteristic (at the antenna coupler) and having an antenna coupler built for multi-band flexibly is important.  

Tuning for low impedance back in the day was called current feeding your antenna and tuning an antenna system that showed a high impedance at the antenna coupler was called voltage feeding.

The above simple, efficient and flexible mulit -band antenna coupling system is controlled in the comfort of your shack.

The link antenna coupler is not limited to center fed antennas, however, it shines when it comes to mulit band center fed antennas.

Chuck
« Last Edit: September 18, 2012, 07:13:43 AM by AA5WG » Logged
KF7NUA
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Posts: 153




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« Reply #6 on: September 18, 2012, 06:47:14 AM »

My previous suggestion was to add the 80m loading coils and extension wires to
the 20m wires rather than the 40m wires.  You can then get all three bands in
the same space as a 40m dipole.

Here is an example (though you don't need to follow all the details):

http://www.hamuniverse.com/ae5jufielddayantenna.html


Dale - I started this thread because I am frustrated. The more I read about traps and coils the more the term "Narrow Bandwidth" kept surfacing and stuck in my mind. I do not want to lose anymore bandwidth than I have too, that may sound selfish but I I do not have the luxury to teardown and try another if I am not satisfied, I am in my 60's, semi disabled with limited funds. Because I live next door to the biggest HOA Nazi leader in my area I also need to set up and tear down my antenna each time I want to use it, so last night I decided that simplicity is a must. This is why the G5RV surfaced in my list, one wire, one length, I would hope more bandwith than the trap antennas and more bands. I feel like a fish out of water, I keep flipping from this to this and dying a slow death.
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KC4MOP
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Posts: 743




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« Reply #7 on: September 18, 2012, 08:48:08 AM »

Here's a link to a company offering a G5RV 102 foot for the bands you are interested.

http://www.antennasmore.com/g5rv.htm

The ad reads  that you need a tuner.

OR string whatever you can get out there as a dipole and use window line to your shack with a tuner.
If you are running legal limit; then things can get complicated.

Fred
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WB6BYU
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Posts: 13336




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« Reply #8 on: September 18, 2012, 09:23:33 AM »

Quote from: KF7NUA

Dale - I started this thread because I am frustrated. The more I read about traps and coils the more the term "Narrow Bandwidth" kept surfacing and stuck in my mind. I do not want to lose anymore bandwidth than I have too, that may sound selfish but I I do not have the luxury to teardown and try another if I am not satisfied, I am in my 60's, semi disabled with limited funds...



I understand.  These are clear limitations you have to deal with.  And when you explain
the limitations you are working under, it makes it easier for the rest of us to make suggestions
that are suitable for you.  There are millions of possible options - the difficult part is figuring
out what best meets your needs.

In your case I'd suggest a doublet of whatever length will fit the space you have available
with twinlead or ladder line to a tuner in the shack.  This would look very much like G5RV,
but without the coax, and could be made shorter if needed.   80m performance drops off
as you make it too short, but it should give you all HF bands without requiring any outdoor
adjustments.  Using twinlead to feed it improves the efficiency on 15m and 10m.


There are other possibilities:  I visited one ham while his wife was out shopping and we
put a wire loop around his house, choosing an insulation color that matched the roof material.
3 months later his wife hadn't even noticed it, even though you could see it out the bedroom
window if you knew where to look.

Are you allowed to put up Christmas lights?  If so, get a couple hams from the local club to
come over and string them up around your eaves, and, while they're at it, attach a wire
antenna to the back side of the fascia boards. 

Unfortunately I'm a bit too far away to come over and put up something for you...
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AA4PB
Member

Posts: 12893




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« Reply #9 on: September 18, 2012, 09:49:05 AM »

"In your case I'd suggest a doublet of whatever length will fit the space you have available
with twinlead or ladder line to a tuner in the shack"

Given your physical limitations, this is what I'd suggest also. You put up the antenna once and you get what you get but the tuner should provide an acceptable match to the radio on multiple bands. Using twinlead or ladder line will minimize the feed line loss even, on those bands where the antenna mismatch is high.
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KF7NUA
Member

Posts: 153




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« Reply #10 on: September 18, 2012, 11:20:55 AM »

Quote from: KF7NUA



I understand.  These are clear limitations you have to deal with.  And when you explain
the limitations you are working under, it makes it easier for the rest of us to make suggestions
that are suitable for you.  There are millions of possible options - the difficult part is figuring
out what best meets your needs.

In your case I'd suggest a doublet of whatever length will fit the space you have available
with twinlead or ladder line to a tuner in the shack.  This would look very much like G5RV,
but without the coax, and could be made shorter if needed.   80m performance drops off
as you make it too short, but it should give you all HF bands without requiring any outdoor
adjustments.  Using twinlead to feed it improves the efficiency on 15m and 10m.


There are other possibilities:  I visited one ham while his wife was out shopping and we
put a wire loop around his house, choosing an insulation color that matched the roof material.
3 months later his wife hadn't even noticed it, even though you could see it out the bedroom
window if you knew where to look.

Are you allowed to put up Christmas lights?  If so, get a couple hams from the local club to
come over and string them up around your eaves, and, while they're at it, attach a wire
antenna to the back side of the fascia boards.  

Unfortunately I'm a bit too far away to come over and put up something for you...

Thanks again

To answer your questions and then some.

Mission style Stucco home with chicken wire embedded.
Flat roof 60%, small raised portion 40% covered with tile
No trees, well maybe 1, it is about 14' tall.
Back yard is 100ft wide and 50ft deep , problem is HOA guard next door built her home set back further  because it is a L shape so the living area of her home is right along side my back yard, hence the reason I wanted to go with a short dipole in the beginning.

I thought I read that the Doublet antenna is 130ft long, this would be too long.


« Last Edit: September 18, 2012, 11:32:25 AM by KF7NUA » Logged
KF7NUA
Member

Posts: 153




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« Reply #11 on: September 18, 2012, 11:37:53 AM »

I also want to thank everyone else that has respondedwith suggestions to try and help.

Any feedback on the so called"Mystery Antenna"
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KQ6Q
Member

Posts: 988




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« Reply #12 on: September 18, 2012, 11:39:08 AM »

Get an Elmer from your local ham club to come out and make suggestions - he/she will be able to ask and answer questions in person, make recommendations that will fit your budget. Depending on constraints, even a Magloop from MFJ or a hamstick dipole setup might be solutions. Some have great success with a screwdriver antenna hidden in a post-mounted birdhouse. with radials in the surface of the yard.  With the wire mesh in the stucco on the house, a wire antenna that runs close to the house might not be your best choice.
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WB6BYU
Member

Posts: 13336




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« Reply #13 on: September 18, 2012, 11:57:58 AM »

Quote from: KF7NUA

I thought I read that the Doublet antenna is 130ft long, this would be too long.




"Doublet" is a generic term that means "two wires of the same length on either side of
the feedpoint", often fed with open wire line or twinlead.  A dipole is a doublet, and a
doublet will be a half wave dipole on some frequency, but the point is that it is used
on multiple bands without too much concern about the feedpoint impedance.

A common approach is to make a doublet a half wavelength at the lowest frequency
(so ~130' for 80m) but it can be any reasonable length:  efficiency is best when it
is at least 3/8 wavelength long, but many hams have use them at 1/4 wavelength
(effectively a 40m dipole operating on 80m) and managed contacts.
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N4CR
Member

Posts: 1672




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« Reply #14 on: September 18, 2012, 01:37:35 PM »

I thought I read that the Doublet antenna is 130ft long, this would be too long.

Doublet is a very broad term as pointed out above.

As far as 130 feet goes, when you are creating a multiple band doublet, one of the things you want to make sure doesn't happen is that the antenna is a full wave or multiple of a full wave on a band you want to use. (because that makes it a high impedance feed point on that band)

It's very easy to make a 80 meter doublet that won't tune on 40 meters for this reason. The classic fix for this is to make it a bit too long for 80 meters. This puts the full wavelength just below the 40 meter band and makes it MUCH easier to tune on 40 meters. For this reason, 135' is a commonly suggested length that doesn't put a full wave length or multiple of full wave length inside any band we are allocated up to 10 meters.

Insulation decreases the velocity factor of wire. It makes dipoles act longer than they are. If you use insulated wire it won't match the design goals of bare wire which has a velocity factor of 1 in earth atmosphere. (I'm sure I've read that bare wire dipoles in earth orbit have a velocity factor greater than 1 because there is no atmosphere)

432/frequency=feet assumes you are using bare wire. If you use insulated wire it can vary in velocity factor based on the wire diameter and the thickness and composition of the insulation. If you take the time to do a measurement, you can come up with a new formula that works for the insulated wire in question.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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