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Author Topic: Doublet vs horizontal loop  (Read 3369 times)
AF5CC
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Posts: 832




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« on: November 06, 2013, 10:52:52 AM »

I currently have a 100 foot long doublet fed with 300ohm twin lead, to a balun, and then with RG-11 coax to the shack. I am getting ready to redo it, but also realize with some extra work, I could convert it into a 200 or so foot horizontal loop. Part of the loop would slope down to a lower height than the rest of it.  It would be in a sort of pentagon shape, and most of it would be 25 feet or so above the ground. 

Would there be any advantage to going to a loop over my current doublet?  I would be using it on 80-10m with a tuner.

John AF5CC
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KB4QAA
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Posts: 2324




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« Reply #1 on: November 06, 2013, 12:15:30 PM »

Loops are omnidirectional.  They tend to be a bit quieter (this statement can start fist fights) perhaps for p-static reasons if nothing else.  It will probably give you more complete near-in coverage due to the high vertical pattern.  It can be easy to match over several bands.  Impedance at resonant frequency is about 200 ohms.

Specific shape is unimportant as long as you don't make it too long like a rectangle.   Its all an approximation of a circle anyhow.

A dipole well below a half wavelength high is pretty much omnidirectional.  Mostly.
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WB6BYU
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« Reply #2 on: November 06, 2013, 01:38:49 PM »

On 80m you probably won't see much difference between the two, even if you
were to make the loop resonant (about 275' or so.)  With 300 ohm twinlead the
SWR on the 75 ohm coax will generally be lower with a resonant loop than with
the doublet, especially with a 4 : 1 balun, which may improve efficiency, but this
depends on many factors.

On 40m the loop resonant loop may not be as good for short skip (out to 100 miles
or so) because the loop has an overhead null.  For other lengths of loop the null
may be less significant.  The impedance seen by the coax will vary significantly on
the doublet depending on the length of the 300 line used, while with a resonant loop
the variations will be much less because the feedpoint impedance is closer to that
of the twinlead.

As you go higher in frequency the maximum radiation from the doublet tends to a
smaller angle from the direction of the wire, while a loop will tend to have a pattern
of lobes and nulls in all directions (probably 12 lobes about evenly spaced around the
compass on 10m with nulls between them.)


So whether it is worthwhile for you to change will depend on how important the actual
radiation pattern is for your preferred style of operation.

If you want the convenience of operating without a tuner, a full wave loop for 80m
(length of ~275', resonant around 3.55 MHz) fed with a 4 : 1 balun to 50 ohm coax
will give an SWR of 2 : 1 or better on many of the HF bands.  With 300 ohm twinlead
to a remote balun the SWR on the coax may be low enough so that the losses in
the coax are reasonable.  (The best way to adjust the loop length is to center the
SWR curve for the 20m band.)

On the other hand, if you can't make it much longer than 200', and you are happy with
the performance of your current antenna, it might not be worth the added effort of
converting the antenna to a loop.  While the gain in a specific direction can be 10dB or
more different between the two antennas, the overall performance isn't all that different.
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KA7NIQ
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« Reply #3 on: November 07, 2013, 09:29:27 PM »

I currently have a 100 foot long doublet fed with 300ohm twin lead, to a balun, and then with RG-11 coax to the shack. I am getting ready to redo it, but also realize with some extra work, I could convert it into a 200 or so foot horizontal loop. Part of the loop would slope down to a lower height than the rest of it.  It would be in a sort of pentagon shape, and most of it would be 25 feet or so above the ground. 

Would there be any advantage to going to a loop over my current doublet?  I would be using it on 80-10m with a tuner.

John AF5CC
W8JI has shown that a loop antenna at low heights, actually has more ground loss then a Dipole, at the same height!

If it were me, and I wanted to "play", I would look at the ZS6BKW antenna. The ZS6BKW has shown to have the best match on the most bands, of any center fed antenna.
However,  a 10-80 meter ZS6BKW suffers a bit on 80 vs a full size dipole.





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W5DXP
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« Reply #4 on: November 08, 2013, 05:38:58 AM »

However,  a 10-80 meter ZS6BKW suffers a bit on 80 vs a full size dipole.

The VK1OD transmission line loss calculator says 40' of ladder-line feeding a ZS6BKW will have about 0.75dB of loss on 80m which is not too bad. Unfortunately, there will be about 2.5dB of loss in 80' of RG-8x so I use a 500pf door-knob cap in series on the coax side of the 1:1 choke (switched in on 80m and shorted out on the other bands) that gives me the following measured SWR curves on the coax. The autotuner in my IC-756PRO does the rest. According to my MFJ-259B, the impedance looking into the series capacitor at minimum SWR is about 30-j15 ohms.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KA7NIQ
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Posts: 255


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« Reply #5 on: November 08, 2013, 07:04:34 AM »

However,  a 10-80 meter ZS6BKW suffers a bit on 80 vs a full size dipole.

The VK1OD transmission line loss calculator says 40' of ladder-line feeding a ZS6BKW will have about 0.75dB of loss on 80m which is not too bad. Unfortunately, there will be about 2.5dB of loss in 80' of RG-8x so I use a 500pf door-knob cap in series on the coax side of the 1:1 choke (switched in on 80m and shorted out on the other bands) that gives me the following measured SWR curves on the coax. The autotuner in my IC-756PRO does the rest. According to my MFJ-259B, the impedance looking into the series capacitor at minimum SWR is about 30-j15 ohms.

Great Information Cecil!
I was told that a ZS6BKW is down on 75/80 because it is short, for that band.
Still, even if it is down on 75/80 a bit, who really cares ?

At the height most Hams will erect it at, it will be a cloud warmer anyway, better for local NVIS communications then DX.

Much NVIS signals are plenty strong enough that a DB or so will not be missed much  Cheesy


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W5WSS
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Posts: 1683




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« Reply #6 on: November 08, 2013, 07:52:40 AM »

Cecil, I have 2- 500pf 20 KV door knob caps brand new.

Where did you locate the installation of the door knob cap and switch network for optional use?

Assuming the cap is in series with the center conductor of the antenna feed line located at the output side of what a choke balun ,tuner, rig, where?

Can you post a picture here?

Thanks 73
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W5DXP
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« Reply #7 on: November 08, 2013, 05:06:38 PM »

Can you post a picture here?

Note that my ZS6BKW is at 40' so 40' of ladder-line drops straight down to my window to two banana plugs mounted in a piece of plexiglass. So here is my setup.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5WSS
Member

Posts: 1683




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« Reply #8 on: November 10, 2013, 07:59:46 PM »

OK thanks! Nice I have some and perhaps I too could utilize the same techniques I see the male to male adapter between the two boxes.

I would do it the same way and yes the picture is how I imagined that you are doing it.

Thanks 73
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