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Dual polarity quad...possible to build?
- Thread starter Broke Spoke
- Start date
Did any of you guys bother to look at or read the links that tecnicoloco posted? If you did you will see that none of these three are really quite as simple as CK would have us believe, and none excepting Joel Moell's (2) article is even close to being connected directly to the coax. I think the gist of this thread is trying to suggest that this can be done simply by connecting the coax to the open ends of a quad port.
George Prichard's article (1) use gamma matchers and a choke of some design.
The manual for the Hy Gain (3) uses 2 wires for the driven element, similar to what Homer suggested. There is also a complicated decoupling device used as well, and the Hy Gain surely is not a simple insulator located at a corner port on the Quad element that separates it and allows for a direct connection to the coax feed line as is being implied in most of this thread.
Simple it is not.
When you charge an open port at point A on a quad that has another open port at point B, 90* degrees away with a feed line directly attached, that feed line will become a direct part of the antenna. It might work and maybe it might show a good enough SWR using a long feed line, but I still suggest that match at the feed point is not what you might imagine.
Here is detail B, in Figure 7, in the High Gain Manual that shows the secret decoupling device that we wouldn't have heard about, unless we bothered to read the fine print...instead of just looking at the pictures and listening to the simplistic ideas talked about here.
View attachment HyGain manual.pdf
I don't mean to be critical of ideas here, but we have to consider the real subject regarding antennas, or be fooled.
George Prichard's article (1) use gamma matchers and a choke of some design.
The manual for the Hy Gain (3) uses 2 wires for the driven element, similar to what Homer suggested. There is also a complicated decoupling device used as well, and the Hy Gain surely is not a simple insulator located at a corner port on the Quad element that separates it and allows for a direct connection to the coax feed line as is being implied in most of this thread.
Simple it is not.
When you charge an open port at point A on a quad that has another open port at point B, 90* degrees away with a feed line directly attached, that feed line will become a direct part of the antenna. It might work and maybe it might show a good enough SWR using a long feed line, but I still suggest that match at the feed point is not what you might imagine.
Here is detail B, in Figure 7, in the High Gain Manual that shows the secret decoupling device that we wouldn't have heard about, unless we bothered to read the fine print...instead of just looking at the pictures and listening to the simplistic ideas talked about here.
View attachment HyGain manual.pdf
I don't mean to be critical of ideas here, but we have to consider the real subject regarding antennas, or be fooled.
They do show a wire rectangular gizmo on the vertical (detail B) connection, but not on the horizontal (detail A) connection. . .
I don't understand any of it, but maybe you only have to fix one port. I was just trying to show that such a deal might not be so simple.
Homer I think you said you did a dual polarity current fed quad, could you describe for us a little of what you did it and how?
No I didn't do it, I started to do it with the last quad, but someone wanted the antenna and I let it go for cost of materials. I said I had seen something like it somewhere along the way. Maybe I'll do one. . . I have talked it over with my friend who lives behind me who has made his share of antennas in the past. He says it works, and isn't complicated. Perhaps that means he's done one . . .
Just use about 6' of 1 3/8 mast for the boom, run it through one of those rotors which allows the mast to go up throught it. Place the rotor on it's side on a mounting plate so it becomes the polarization adjustment, then drive your full wave wire quad with an electrical 1/4 wave of RG-11. (about 7' 2") - Use the velocity factor x 108" for the measurement.
For local talk you'll probably always keep it vertical but for DX you'll probably want to keep it horizontal.
For local talk you'll probably always keep it vertical but for DX you'll probably want to keep it horizontal.
gee, I dunno, this is not really that complicated, just use a BALUN @ the feed points
The feedline is connected to a corner which gives a horizontally polarized transmitted signal. Received signals of horizontal polarization induce currents in the loop that go down the feedline to the receiver. Vertically polarized signals induce currents within the loop also, but their magnitude and phases are such that they cancel out.
RF current from the transmitter is highest at the feed point There is another transmitting current maximum at 180 degrees. At 90 and 270 degrees, current is at a minimum. Theoretically in a perfect quad, loop current would be zero at these points. In a practical beam, it is less than 1 per cent of the maximum.
If there is indeed no current at 90 degerees, why couldn't we just break the loop there, If you have antenna analysis software , try this : Insert a very high value resistive load in series at point 270 degrees in the DE of your favorite diamond-shaped quad model for any band. If the quad is symmetrical, then the resonant frequency, pattern and feedpoint impedance will show very little change.
With the added break, we have the driven element being fed at 0 degrees gives horizontal polarization, while feeding it at 90/270 degrees gives vertical polarization. Both modes can be handled by the quad simultaneously. So why not just hook a feedline to each point and switch from one to another at the receiver?
Breaking the loop and adding a 50-ohm load at a current minimum will not affect it, but adding an unbalanced feedline certainly will. The braid of the coax will become part of the antenna. As an EZNEC exercise, hang a 6-foot wire (representing the coax shield) at a current null point in your favorite quad DE and watch the pattern change for the worse.
Fortunately, current flow on the outside of the coax can be choked off to restore the current minimum. A balanced-to-unbalanced transformer (balun) will do it. RF current will be confined to the coax inner conductor and the inside of the shield, and will remain near zero for cross-polarized signals. For the unused polarization mode, it is almost as if the coax isn't there at all!
they were feeding a resonant antenna at port "A" with a tuned feedline,
and feeding port "B" with an additional 1/4 wl (to shift it it an additional 90 degrees and change to polarization) which changed the resonant freq.,...
so,......they then they ADDED a simple beta match @ 90 degrees to bring to BACK to resonance
You may be right hookedon6.
Right now my 2 element quad has one feed point and shows 1.15:1 match. but when I add the 2nd feed point at the other corner the match goes to heck in a hand basket, showing 3.038:1 at port 1 and 2.88:1 at port 2. See below.
This stuff is above my level of understanding with modeling or real world, but I worked with a friend once trying to make his quad with only one driven element wire into a dual polarity setup, and when we connected the coax direct, we got bad results and when we removed one of the feed lines the match changed again, but still not good. We felt that allowed us to understand that the feed line on the opposite side affected the match, and we assumed that with the line attached in such a way, it naturally became a part of the antenna as a consequence.
I understand a little of the ideas you posted, but a little understanding is not enough. That is why I asked Homer about his attempt to do the same, but I must have misread his words.
View attachment Dual Polarity Quad idea..pdf
they were feeding a resonant antenna at port "A" with a tuned feedline,
and feeding port "B" with an additional 1/4 wl (to shift it it an additional 90 degrees and change to polarization) which changed the resonant freq.,...
so,......they then they ADDED a simple beta match @ 90 degrees to bring to BACK to resonance
The example I was talking about here was the Hy Gain beam if it makes any difference. It had a separate wire for each polarity making the driven element a dual wire setup, and that may be different from how a single wire quad would work...with two feed lines attached 90* degrees apart.
I have been thinking of just that exact idea for a 2m quad I was thinking about building.The old Alliance U-100 rotators were perfect for this as they allowed the mast to pass straight thru the rotator. Turned over on it's side it allows the boom to pass thru and be rotated 90 degrees for polarity changes. One feedline and two polarities by rotating the boom to switch polarities. The U-100 rotators are old but there were thousands in use and are still around.
warrior369
W9WDX Amateur Radio Club Member
PDL2
I attach the US patent for the PDL2. Loop size and element spacing for the driven and director elements can be found in the patent.
The driven element of the PDL2 has a full wave of wire. The four 'spreaders' of the driven element plus the 'basket' on which the separate gamma matches connect, form two (sort of) electrically isolated folded dipoles. Each dipole is orientated in the vertical or horizontal plane. Each dipole end connects directly to the loop of wire at current maximas. The half wave dipole length is made possible by ensuring that the overall length of the two 'spreader' elements for each polarisation are approximately 70.7% with the remaining 29.3 % found in the 'basket'.
W8JI says that if you attempt to get vertical and horizontal polarisation by feeding at wire corners you will come unstuck because one of the polarisations will present a very high impedance.
Thats all I know about the PDL2 because its operation is way above my understanding.
I tried to model the PDL2 within EZNEC but came unstuck because I do not know how to model vertical and horizontal feeds and their connection into a loop of wire. Has anyone tried to model this or even better, is the EZNEC file available?
I attach the US patent for the PDL2. Loop size and element spacing for the driven and director elements can be found in the patent.
The driven element of the PDL2 has a full wave of wire. The four 'spreaders' of the driven element plus the 'basket' on which the separate gamma matches connect, form two (sort of) electrically isolated folded dipoles. Each dipole is orientated in the vertical or horizontal plane. Each dipole end connects directly to the loop of wire at current maximas. The half wave dipole length is made possible by ensuring that the overall length of the two 'spreader' elements for each polarisation are approximately 70.7% with the remaining 29.3 % found in the 'basket'.
W8JI says that if you attempt to get vertical and horizontal polarisation by feeding at wire corners you will come unstuck because one of the polarisations will present a very high impedance.
Thats all I know about the PDL2 because its operation is way above my understanding.
I tried to model the PDL2 within EZNEC but came unstuck because I do not know how to model vertical and horizontal feeds and their connection into a loop of wire. Has anyone tried to model this or even better, is the EZNEC file available?
Have you considered circular polarity? It would use crossed dipoles fed 90 degrees out of phase. The reflector and directors can be regular square loops. The advantage to circular would be during skip conditions when the signal polarity could be rapidly changing, the CP will respond much better and should reduce apparent signal fading. The same should happen on the receive end. Total ERP or gain would be half, but reception should be greatly improved.