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antenna project pdl-II made into a quagi please help antenna qurus!

Shockwave

Sr. Member
Sep 19, 2009
3,785
3,289
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I see another one of my favorite antennas is being discussed here. About 15 years ago I built a PDL-4 out of a PDL and a shooting star. Sorry to say at the last minuet I allowed myself to be deceived into using a field strength meter in the near field for tuning its gain and ruined that project quickly. Impedance changes fairly drastically when adding other parasitic elements so you can expect to have to retune those gammas. I'm in the process of building a PDL-3 with one set of Yagi directors as we speak and had been considering the Quagi versus Quad ideas.

With 3 elements is should be close to the same gain Quagi or Quad. There was some talk about how the Yagi directors may interact with the folded dipole portion of the driven element in a negative way and that just may be the case. First, lets understand the term "folded dipole" is only being used here because the 1/2 wave dipole is not inline. The real folded dipole would consist of an electrical full wavelength of conductor, folded in half, over itself. What we really have on the PDL is more of a loaded dipole that is an electrical 1/2 wavelength but the loop inductors shrink its physical length to fit the quad loop around it.

The next thing to notice is it's the vertical dipole that feeds the horizontal loop and the horizontal dipole that feeds the vertical loop. If the dipoles radiated significant energy, the PDL might have less then 3 db isolation between vertical and horizontal since the dipole and quad would always be radiating in the opposite polarity. That is not the case since the dipole is delivering most of its energy to be radiated on the quad loop. However, we also have to consider there is most likely some small degree of radiation from the dipoles. That could make it a poor choice to add a Yagi element directly in front of a PDL driven element.

The Yagi is more likely to parasitically couple to the energy in the dipole of the driven element rather then the energy in the quad of the driven element. That energy is in the reverse polarity and also takes away some energy that would be driving the quad driven element to parasitically excite the wrong Yagi director. For a PDL-3 or even a 4, I would probably go all quad. Anything larger and I would make the reflector, driven and first director as quads and all other directors as Yagi's. That's because each director you add is excited by the element in front of it. As long as you have one quad director in front of the PDL driven element, any Yagi directors in front of it should only be parasitically coupling to the desired polarity.
 

IRock817

Active Member
Oct 29, 2011
168
32
38
Shockwave- good luck with the three element! That was my first choice, but we came up with two four element booms so we're going four. The guy that designed these runs a four element. He lives 30 miles away from my area. When he keys (barefoot) nobody can believe it. Puts me in the red every time. When he spins the house around he drops out to where we can't hear him and the needle relaxes too the left.

Damn this is fun **Jump_im**
 

sob

kc7max
Apr 1, 2012
15
2
13
[


The next thing to notice is it's the vertical dipole that feeds the horizontal loop and the horizontal dipole that feeds the vertical loop. If the dipoles radiated significant energy, the PDL might have less then 3 db isolation between vertical and horizontal since the dipole and quad would always be radiating in the opposite polarity. That is not the case since the dipole is delivering most of its energy to be radiated on the quad loop. However, we also have to consider there is most likely some small degree of radiation from the dipoles. That could make it a poor choice to add a Yagi element directly in front of a PDL driven element.

The Yagi is more likely to parasitically couple to the energy in the dipole of the driven element rather then the energy in the quad of the driven element. That energy is in the reverse polarity and also takes away some energy that would be driving the quad driven element to parasitically excite the wrong Yagi director. For a PDL-3 or even a 4, I would probably go all quad. Anything larger and I would make the reflector, driven and first director as quads and all other directors as Yagi's. That's because each director you add is excited by the element in front of it. As long as you have one quad director in front of the PDL driven element, any Yagi directors in front of it should only be parasitically coupling to the desired polarity.[/QUOTE]

Hi Shockway, thanks for sharing with us and yes i understand that its a folded 1/2 wave dipole.Im trying to make sense of you saying the vertical is feeding the horizontal dipole and visa - versa.Im looking in the pdl2 manual and fig 9 shows the vertical connector , the vertical gamma match feeding the vertical folded dipole. ??? . also thanks again for all your info , it sure helps the thread.
 

hookedon6

W9WDX Amateur Radio Club Member
Jun 21, 2008
1,484
243
73
from what i understand the only reason the quad out performs the yagi is the driven element of the quad has more gain....


"the driven element of the quad has more gain...." that a debateable issue,.... depending on the number of elements]

actually, the quad is a much quieter RX than a yagi
 

Shockwave

Sr. Member
Sep 19, 2009
3,785
3,289
273
I've also taken a closer look at my antenna and to my surprise, the PDL quad with its dipole driven element is vastly different then other quads. All other quads feed the signal into one side for vertical polarization and into the bottom (or top) for horizontal polarization.

Something very interesting starts to happen when you drive the quad through a half wave. The feedpoint for the half wave is low impedance but the ends that drive the quad are high impedance. This allows the horizontal dipole to feed the quad at the sides while the radiation currents in the quad are still orientated for horizontal polarization. Just as though it were driven at the bottom without first passing through the half wave section.

Now it makes sense that the radiation currents of the dipole and the quad driven element are reinforcing each other to provide the increased gain we do see with this design. Why was it never taken past 2 elements is a mystery. Perhaps it has to do with patents held on the matching network or Avanti not wanting to create competition for their easy to assemble Moonraker series.
 

Marconi

Usually if I can hear em' I can talk to em'.
Oct 23, 2005
7,176
2,224
343
Houston
I've also taken a closer look at my antenna and to my surprise, the PDL quad with its dipole driven element is vastly different then other quads. All other quads feed the signal into one side for vertical polarization and into the bottom (or top) for horizontal polarization.

Something very interesting starts to happen when you drive the quad through a half wave. The feedpoint for the half wave is low impedance but the ends that drive the quad are high impedance. This allows the horizontal dipole to feed the quad at the sides while the radiation currents in the quad are still orientated for horizontal polarization. Just as though it were driven at the bottom without first passing through the half wave section.

Now it makes sense that the radiation currents of the dipole and the quad driven element are reinforcing each other to provide the increased gain we do see with this design. Why was it never taken past 2 elements is a mystery. Perhaps it has to do with patents held on the matching network or Avanti not wanting to create competition for their easy to assemble Moonraker series.

Shockwave, I too never realized that the PDLII was voltage fed either.

Since I've never heard such a difference described regarding the PDLII, I can only assume that very few owners ever wondered about their PDLII's feed point locations being reversed from the typical quad that is current fed.

If you check close again, you will also find that basically the PDLII is very similar to the Signal Engineering Lighting X, line of beam antennas.

IMO, there has to be some RF cancellation in the 1/2 wave matching network that allows mostly for only the quad wires to radiate. If I'm right maybe Bob could put his EzBob to work and help us understand how the currents might be flowing, that allows these voltage fed antennas to work without these very large feeders from radiating excessively, and messing up the nice quad pattern that is possible.

The big advantage using voltage feed is it allows for a single quad wire to be used as a dual polarity radiator, which is obvious with both of these antennas and very different from the more traditional current fed quad.
 
Last edited:

sob

kc7max
Apr 1, 2012
15
2
13
What happened to ya SOB?

Bump:eek:

sorry irock been really busy, havnt had a chance to play.how bout you?I am liking some of the post though .Its helping me understand how the pdl2 works!still debating on building a 4 element true quad or using the pdl2 with 2 quad elements in front.it would be cool to have the 2 antennas side by side for a shot out.which one do you think would prevail??? front to back? side to side ect.? anybody?
 

IRock817

Active Member
Oct 29, 2011
168
32
38
I'm with ya, on the no time for the hobby's! I did have one of my buddy's put up a three element PDL this past weekend! We did an across town-(about 20 mile) radio check and his antenna is working real well. Flat swr and tight. He's happy with it and says it works allot better with the added director. Hears and talks better then stock, he said!

Have fun all!
 

sob

kc7max
Apr 1, 2012
15
2
13
I'm with ya, on the no time for the hobby's! I did have one of my buddy's put up a three element PDL this past weekend! We did an across town-(about 20 mile) radio check and his antenna is working real well. Flat swr and tight. He's happy with it and says it works allot better with the added director. Hears and talks better then stock, he said!

Have fun all!

you know what would have been really cool would have been to have before and after field strength tests on your buddys pdl2/pdl3
 
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IRock817

Active Member
Oct 29, 2011
168
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My four element will get the full array of tests when we put it together. The tech says it will take two days to tune. Now that's some fine tuning!:)
 
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Marconi

Usually if I can hear em' I can talk to em'.
Oct 23, 2005
7,176
2,224
343
Houston
My four element will get the full array of tests when we put it together. The tech says it will take two days to tune. Now that's some fine tuning!:)

817, was your Tech bragging or complaining?

A well designed antenna can make the difference, and otherwise some antennas just don't tune up well regardless of what is done or the time spent trying.

Can you discuss a little what kinds of tests are planned for your new antenna?
 

IRock817

Active Member
Oct 29, 2011
168
32
38
He'll make sure it's resonate, swrs are flat and the field strength meter is happy! Nothing special, we want it the best it can be when we put it up for good. He's put together several of these, but as you know no two are alike even with store-bought. With each application they all need fine tuning.
 

sob

kc7max
Apr 1, 2012
15
2
13
hey irock , did you get your antenna up??? i still havnt but i did buy a toy that will help when i do.its a bucket truck that reaches close to 30 feet.
 

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    I hope he did not ask that question on the "ZED" he would get vaporized...75% telling in some "polite way" are you going to put up a "REAL" antenna? The rest telling him that's the greatest antenna ever with my 7300o_O:coffee:
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