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Ideas for stealthy horizontal 144mhz antenna?
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by NZ5N on July 13, 2007
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Due to neighborhood antenna restrictions, I've been limited to HF operation with a vertical hidden by a palm tree. However, my European vaction has whetted my appetite to start chasing grid squares again on 2 meters.
Any ideas for a stealthy 2m horizontal antenna? Even a small yagi is out of the question (except perhaps in the attic), maybe stacked wire dipoles or halos I could hide in a tree?
Any thoughts would be appreciated.
73,
Bill NZ5N
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by K4SAV on July 13, 2007
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Since you mentioned it, how about stacked 2 meter dipoles. Two of these spaced 4 feet apart (over/under) fed in phase with the top one at 20 feet will deliver about 12.4 dBi of gain. It is bi-directional and the lobes are very wide in the azimuth direction. Another two of these placed at right angles to the first pair, switch selectable, and you can cover 360 degrees azimuth with no rotator. A lot of performance for a very simple antenna.
Jerry, K4SAV
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by NZ5N on July 13, 2007
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That sounds doable, thanks. What about 3 stacked? Would it be fed with a phasing harness?
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by K4SAV on July 13, 2007
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Yes you can get more gain by adding more elements, but this will complicate the antenna. There is one good feature of this configuration, adding more elements does not change the lobe width in the azimuth direction. For all these stacks the lobe width is the same as for a single dipole, so two sets is all you need to cover 360 degrees with no rotator.
For a stack of 3 with the top element at 20 ft and elements spaced at 5 feet, the gain should be about 14.3 dBi.
It's interesting that for both of these (either a 2 or 3 stack), the matching can be the same. For a 50 ohm match you can use 1/4 wavelengths of 75 ohm coax. Actually you will need to use 3/4 wavelengths because 1/4 won't reach.
For a four stack, you will have to move the antenna higher to realize any more gain, because the lower antenna will be too close to the ground. The matching network for this antenna will be different.
Note that all the gain numbers I gave are not free space gain. They include ground gain at the stated height.
Jerry, K4SAV
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by NZ5N on July 14, 2007
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Thanks again, think I'll start with a simple 2-stack and see how it works before trying something more complicated. Will simple dipoles do the job or will I need gamma matching, as indicated in Fig.7 of Chapter 18 in the ARRL Antenna Book (18th ed.)?
As you know, I had success with Delta Loops in Europe. Instead of dipoles, what about vertically stacking 2 horizontally polarized loops? Would that work any better than dipoles? If each loop has 100 ohms impedance, wouldn't a parallel feed give a good 50 ohm match?
I shouldn't get my hopes too high, any type of 2 element antenna will be far from a cannon on 144 mhz, but maybe I'll be able to make a few contacts. I'd like to have enough to work scatter with WSJT. The author of this software has written, "If you have 100 W or more to a modest Yagi up at least 40 feet, then with the help of WSJT you should be able to work similarly equipped stations in the 500-1100 mile range at nearly any time of the day or year. (On the minimum end of the scale, WSJT QSOs have been made with as little as 10 W)." My antenna won't be 40' high and it won't be a "modest Yagi," but perhaps with my 150 W I'll be able to make some of those "minimum end" contacts.
73,
Bill NZ5N
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by K4SAV on July 14, 2007
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I don't think loops will work any better than simple dipoles, and it will certainly be more complicated. I haven't modeled it though.
I'm sure you have seen UHF antennas stacked this way, using folded dipole elements, and that may be useful with some configurations to allow simpler matching networks. However for a simple 2 stack, you can't get a much simpler matching network than a 3/4 wavelength of 75 ohm coax. You don't need a gamma match. Just wire them up like dipoles and connect a 3/4 wavelength of 75 ohm coax to each one. Connect the other ends of the 75 ohm coax together and connect that to a 50 ohm line. Use the extra length of the matching section to form a choke at each dipole's feedpoint.
Don't sell this antenna too short. Its gain is only down about 2.1 or 2.2 dB from a three element Yagi at the same height, and you don't need a rotator with this antenna. The signal to noise ratio on receive with this antenna won't be as good as a Yagi because of the very wide lobes, but you won't be missing stations because you aren't pointed at them.
Jerry, K4SAV
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by NZ5N on July 15, 2007
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Thanks, I have started working on it, and hope to have it up in time for the CQ VHF contest next weekend. The spacing between the dipoles should be 5/8 wave, correct? You mentioned 4' in one post and 5' in another.
I'm not selling this antenna short, but a 3 element yagi at 20' is not much at 2m, and this is more than 2db worse than a 3 element yagi? Well, we have a to work with what we have. Would I be better off with a 10 element yagi in the attic?
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by K4SAV on July 15, 2007
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The optimum spacing depend on the number of elements. 4 ft was for 2 elements, and 5 ft was for 3 elements. Actually for the 2 element antenna, the optimum gain does peak at 5/8 wave spacing. Here are some EZNEC numbers
Spacing Gain
3 ft ____ 11.14
4 ft ____ 12.48
4.15 ft __ 12.58
5 ft ____ 12.45
6 ft ____ 11.21
Anything above 4 ft spacing and the antenna starts to develop some high angle lobes, which is the reason I chose 4 ft. Look at this document on W8JI's website to understand a little more about spacing.
http://www.w8ji.com/stacking_broadside_collinear.htm
As far as a 10 element Yagi in an attic, I can't answer that because I know nothing about your attic. I have found that high gain antennas in an attic detune badly depending on what they are pointed at. So when you rotate it, the SWR goes all over the place. I tried a 4 element 2 meter quad in an attic once, and it was much worse than a J pole that was mounted on the roof just 6 ft above it.
Jerry, K4SAV
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by KE4DRN on July 16, 2007
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hi bill,
take a look at the Moxon for 2m,
http://cebik.com/moxon/moxbld.html
direct fed 50 ohm coax, takes up less space
then a 2 el yagi but similar gain.
Horizontal or Vertical mounting,
nobody will notice it after it is up.
73 james
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RE: Ideas for stealthy horizontal 144mhz antenna?
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by NZ5N on July 16, 2007
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Thanks James,
I had thought about a Moxon but it's highly directional and I won't have a rotator. For that reason, I also thought about stacked halos, which are omni-directional, but it has less gain than stacked dipoles.
Jerry, it looks as though I'll have plenty of room, if I want to go to a 3-stack do I just run 3 3/4 wave pieces of 75 ohm coax to a common point and then run 50 ohm coax from that point to the shack? Or is the 3-stack feed more complicated than that? In any event, I want to get the 2-stack working before getting more ambitious. Also, is there any reason why I could not make the top dipole a fan dipole with an additional 6 meter element?
73,
Bill NZ5N
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