Evaluating the VHF half-square

[nanZor]

Not content to leave well enough alone and with some time on my hands, I basically bent the full-wave dipole into something different:

http://www.radioreference.com/forum...l-wave-vhf-vertical-dipole-working-great.html

I created a vertical half-square by bending this antenna horizontally with the 1/4 wave ends hanging down. The feedline is now at the top corner, the 1/4 wave element from the braid hangs straight down. The long wire connected to the center conductor runs horizontally for a 1/2 wavelength, and the last 1/4 wave section hangs down again. Kind of looks like a 2-element beam.

(Pictures don't do this justice as it is currently made out of white zip-cord hung in front of white walls, with temporary hookups.

I haven't modeled this or analyzed it any further, so if someone wants to run with it - great!

The thing I didn't notice until I hung it up is that I can utilize the common-mode braid current as a third element! Two-antennas-in-one depending on where you choke the feedline!

As it stands now, the feedline is choked at the corner feedpoint. BUT, if I remove the choke, and run the feedline away from the corner for a half-wavelength, and let the rest of the feedline hang down vertically, I now have what looks like a 3-element beam.

I'm now playing with putting the choke 3/4 wave away from the feedpoint down on the vertical section of the feedline to emulate a 3rd wire.

Well, this seems to be a bit esoteric, especially with what looks like three 1/4 waves inline with each other - expect a somewhat high look with gain - where - I don't know yet.

At least it is easy to build and tear down so there's no evidence.

Update: oops - it might be better to have the horizontal section become the bottom of the elevated half square rather than being at the top -- more testing....

 

[nanZor]

Specialized use only with half-square

I've spent some time with this half-square, and from what I can tell not many scanning enthusiasts would need to build it. (Unlike the very useful full-wave dipole for those of us down low)

It has a nice figure-8 pattern to it, and the look angle seems to be very low. The low look angle does me no good since I'm surrounded by other houses and other obstructions.

If there is any gain in the figure-8 lobes, I couldn't tell. Perhaps because of my low mounting position and the fact I don't have a rig for scanning with a decent S-meter.

About the only thing I can think of that might be useful with this antenna for scanning enthusiasts might be to use the directivity to help null-out a tower in a simulcast system where you are located between the two and get a lot of phase noise, while still allowing for generally good scanning broadside to the antenna.

For now, I bent everything back into a straight vertical full-wave dipole. The half-square will go into the bag of tricks book.

 

[Uplink]

nice job!, any pics? lots of info on a great homebrew scanner antenna here, works great in the attic...

Not content to leave well enough alone and with some time on my hands, I basically bent the full-wave dipole into something different:

http://www.radioreference.com/forum...l-wave-vhf-vertical-dipole-working-great.html

I created a vertical half-square by bending this antenna horizontally with the 1/4 wave ends hanging down. The feedline is now at the top corner, the 1/4 wave element from the braid hangs straight down. The long wire connected to the center conductor runs horizontally for a 1/2 wavelength, and the last 1/4 wave section hangs down again. Kind of looks like a 2-element beam.

(Pictures don't do this justice as it is currently made out of white zip-cord hung in front of white walls, with temporary hookups.

I haven't modeled this or analyzed it any further, so if someone wants to run with it - great!

The thing I didn't notice until I hung it up is that I can utilize the common-mode braid current as a third element! Two-antennas-in-one depending on where you choke the feedline!

As it stands now, the feedline is choked at the corner feedpoint. BUT, if I remove the choke, and run the feedline away from the corner for a half-wavelength, and let the rest of the feedline hang down vertically, I now have what looks like a 3-element beam.

I'm now playing with putting the choke 3/4 wave away from the feedpoint down on the vertical section of the feedline to emulate a 3rd wire.

Well, this seems to be a bit esoteric, especially with what looks like three 1/4 waves inline with each other - expect a somewhat high look with gain - where - I don't know yet.

At least it is easy to build and tear down so there's no evidence.

Update: oops - it might be better to have the horizontal section become the bottom of the elevated half square rather than being at the top -- more testing....

 

[nanZor]

No pics since I don't have much contrast - white wire against white walls in the "shack", er living room.

Imagine two vertical quarter wave sticks with a one-wavelength horizontal wire connecting the two at the bottom. At one corner of the one of the sticks you directly attach the center conductor to the first stick, and the shield to the horizontal wire. You could mount it with the horizontal wire at the top of the half-square, or at the bottom of the half-square shape, ie two sticks facing up or two sticks facing down.

Aside from not being super-practical for general scanning, (it has an azimuth figure-8 pattern where the main lobe is broadside to the two sticks) it might make an interesting antenna up in a very small crawlspace where a full-sized vertical dipole would be out of the question.

 

[nanZor]

Here's a little something showing the quarter wave verticals, the horizontal wire, and it is fed at the intersection of the bottom of one of the the verticals and horizontal wire as seen by the red dot. (Note - this source should be at the exact corner - you can get that kind of precision in the non-Demo version of EZnec) I prefer attaching the center conductor to the horizontal element and the shield to one of the vertical elements.

The azimuth plot shows some gain, but for me the biggest draw would be to put some offending system or other noise in the null. Being quarter-wave elements, the elevation look angle is a bit high - (I'd want to balance those lobes a bit better too) but for me that's perfect.

The swr plot shows that I'm in the ballpark, but I'd have to do real-world adjustments. Not bad though for just bending the full-wave vertical into a different shape and hanging it up with temporary means.

 

[nanZor]

Imagine two vertical quarter wave sticks with a one-wavelength horizontal wire connecting the two at the bottom.

Sorry about that description guys - the horizontal wire is a half-wave, whereas the two quarter wave sticks on each end add up to a total of a full-wavelength of wire overall.

And don't forget to choke the feedline to help maintain the ideal pattern if you can't get a null. For this single-bander for general aviation use, I just find a medium sized plastic water-bottle 2-3 inches in diameter, and wrap about 4-6 closely-spaced turns of the feedline around it. Tape or tie-wrap it up, and remove the bottle if desired. Or use ferrites if you know they are effective at this frequency.

This also makes a good antenna for direction-finding by using it in reverse! Since the nulls are much sharper than the main lobes, instead of trying to achieve maximum signal strength, put the desired station into a null and then look broadside to the two sticks. Your station will lie along that plane in either two directions - but if you know the general direction from which it comes you can ignore the other side.

On a serious note, you could cobble one of these together on a broomstick letting the quarter wave elements hang down, and save someone's life by finding the ELT transmitter.

 

[N_Jay]

Seems like a big "why?".

Most signals are at or near the horizon, and your antenna is kind-of bad there.

 

[nanZor]

In the city or in suburbia, (or indoors behind walls filled with metallic insulation) what is at the horizon are major obstructions to anything coming in from the horizon, and the properties of a nice half-wave dipole which looks predominantly at the horizon are wasted. Rasing the antenna sure helps, but....

Most of my listening comes from above, that is aircraft or hilltop repeater systems above me.

If I was doing simplex work, or doing horizon to horizon work out in the plains, I'd definitely want the lower lobes of a regular half-wave.

I guess the thing is to match the antenna elevation angles to your location and system requirements. It is also the reason why I don't run anything but quarter-wave sticks on my car - here in the city - a higher-gain antenna with lower look angles only puts (or receives) energy coming into or out of storefronts and building entrances.

The beauty of prototyping with nothing but zip-cord means one can experiment easily and see. There are always exceptions to the rule - I might find a spot here mounted low in the backyard where a half-wave dipole might sneak through some obstructions and be absolutely perfect!

It's one of those things - sometimes the textbook solutions may not be appropriate all the time for a particular situation.

 

[KDOXF]

Late to the show...

After reading up on Cebik's site years ago, I set out to construct a 3 element half square beam for 161 MHz. Due to deed restrictions, antennas of any size need to be in my attic so I spent the first few years at my house tinkering with various antennas trying to maximize reception. A large metro area is just to my north and I am most interested in coverage to the south so a directional antenna suits me just fine, still having coverage of most of the metro area off the backside of the antenna.

Attached is a photo of my version, just using 1/2" copper pipe and caps, 1/8" brass rod and PVC for a beam. Mounting to the PVC beam is with nylon hardware. Not robust enough for outdoor use but just fine for the attic. Performance wise, it out paces both a 4 element quad and Arrow Antenna 162-4S Yagi, at least in the attic environment.

 

[prcguy]

Without firing up EZNEC I would think the max current would be near the horizontal to vertical bends, therefore lighting up both horizontal and vertical polarities. I would also think it would be fairly high impedance at the feedpoint (location 1 on the EZNEC demo?) and similar to two end fed half waves fed in parallel.

If it is operating as a dual pol antenna there would be some wasted energy in H pol.
prcguy

 

[KDOXF]

Half Square Beam

Found a copy of Cebik's work on these - QSL Link

 

[nanZor]

Holy cow! 5 year old thread, but since I started it, Bravo!!

Seriously, good work on that beam. You took it to the next level for sure!