Read forum posts about 5/8 verticals coexisting in harmony over horizontal beams, but have not found references to an SPT-500 above an SY 27-4. I contemplate this set-up on a 40' tower but worry that the ground radials on the SPT-500 (which would extend parallel & above the beam's elements) may cause changes of the radiation pattern (or other ill effects) on the signal of the SY 27-4. Is this a viable array or do I have a legitimate concern? Is there a ‘minimum’ height to maintain the SPT-500 above the SY 27-4?
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Super Penetrator 500 above a Sirio SY 27-4?
- Thread starter Wire Dawg
- Start date
The horizontal radials of the vertical antenna will affect the tune of the horizontal beam. Guys use the SY27-4 with an Imax or A99 w/o the radials. Then it is no problem. Dunno if you can use your Super Penetrator 500 on top. Might not work right w/o the radials.
I use a Sirio Gain-Master above a home made 4 element Yagi and it works very good. The Gain Master was up several months before the Yagi was added to the same mast about 7 feet below the omni. I could not determine any difference in the performance of the Gain Master and it is the more sensitive of the two antennas to other objects being too close. I strongly suggest you avoid the use of any omni that has radials above a horizontal beam. The radials will become parasitic elements and alter the pattern and performance of the beam.
I use a Imax 2000 above my horizontal Sirio SY27-3. It works great.
Now I wish I would have went with the 27-4 4 ele. It was only like 10 dollars more. Maybe next time.
Now I wish I would have went with the 27-4 4 ele. It was only like 10 dollars more. Maybe next time.
Thank you for the inputs. Worst case scenario would be having to lower one of the antennas, so I guess I will try it. It would be nice to get the technical scoop as to why it would or would not be advisable. I understand the comment about the radials on the vertical becoming parasitic elements, however, I'm still curious as to whether there is a 'safe' distance to avoid such interaction. I'd like to ask binrat what model of a 5/8 antenna were you using at the time.
It was an old ratshack antenna with a matching coil in the bottom. I replaced the coil with another matching system similar to a V58 / V5000.
If your priority is good performance from the beam, I wouldn't even consider hanging another two resonant elements just 10 feet above it. The first noticeable sign of interaction between the two antennas will be a loss of rejection on the beam. How much interaction there will be is hard to estimate and will have a lot to do with the orientation of the radials in respect to the beam. Why even risk it when there are several antennas that would be ideal to place above a beam? Some will even outperform anything with radials and have less wind load on that rotor.
I forgot that EZNEC could help us predict the effects and since I already had a 4 element Yagi file, it was easy to install the half wave radials 10 feet above the beam. I admit it did not do what I expected. Well under a half db of change is noted with forward gain and rejection. What was completely unexpected is the downward radiation increases by more than 3 db or double what we have without the radials above the beam! By adding the radials you just placed additional reflectors above the beam. That could cause a significant increase in RFI around the neighborhood.
SW, here is a Yagi 2 element I had with a SP 500 above it. It originally had no radials, so I removed it an added a simple model of a 5/8 wave radiator with 4 x 9' radials 8' above it...to see what Eznec might predict.
I don't think I see the same results you did. It looks to me adding the SP 500 above this model did have some effect, but I don't think anybody could tell the difference with these differences noted, and just using their radios about like Binrat told us in his story.
I agree with you hear that there was not much difference in the gain or angle, but I don't see the downward radiation increase you note and I don't see the reflector effect either. I see the presence of the GP seeming to press down some on the pattern OK, but I don't see the potential increase in ill-effects you note for the neighborhood. Both models don't seem to have much indication of currents on the mast either.
View attachment Yagi 2 with SP 500 above and not..pdf
I also did these two models with the ground plane being the active antenna, and there is not much difference here either. However, in this case the pattern looks a little better for the GP with the yagi 2, removed, but again there is no difference in angle and a very small difference in gain.
View attachment Yagi 2 with the SP 500 active. .pdf
BTW, Robb you are right the GP does effect the Yagi 2 match a little, but not as you could likely tell, and this source data is read at the feed point in the models not at the radio end.
SW, if you want, I can email you the models, I saved the work.
SW and Marconi: I appreciate your sharing of the EZNEC models. I wish to avoid RFI like the plague. The question that I have after reading these posts is whether the difference in outcome between the two EZNEC models may be simply attributable to the uses of a two element yagi versus a 4 element. I would prefer to maintain the integrity of the beam.
You won't see any downward radiation in EZNEC with the antenna this close to the ground. Try it in free space. When I did it 36 feet over real ground and shortened the radials a little, I could see an increase in radiation directly upwards by 3 db. When you're in free space you will notice that the radials above the beam cause the pattern to tilt.
Depending on the length and location of the radials you can tilt the front lobe up while pulling the rear lobe down or vise versa. That's because the parasitic elements the radials make up are not in the same horizontal plane as the beam. The current you see exciting the radials will now skew the pattern so that it's tilted. This same effect occurs when vertically stacking elements if they are not all in the same vertical plane.
Depending on the length and location of the radials you can tilt the front lobe up while pulling the rear lobe down or vise versa. That's because the parasitic elements the radials make up are not in the same horizontal plane as the beam. The current you see exciting the radials will now skew the pattern so that it's tilted. This same effect occurs when vertically stacking elements if they are not all in the same vertical plane.
Placing a reflector below the beam would just push a portion of your power straight upwards (unless it was at least 1 wavelength below simulating conductive ground). This characteristic does not appear to have the ability to be manipulated in a useful manner. For example, even though we can cause distortion in the pattern by placing horizontal radials near the beam, this does not seem to have any effect on the angle where peak gain occurs or the "TOA".
Marconi's model shows there is parasitic excitation taking place on the radials. This current is low in magnitude, no more than 20% of the maximum currents. Nonetheless, we have just connected a dipole to the beam. Almost as though there were a coax between the two. It would also be interesting to note any changes this would have in impedance, bandwidth and VSWR. This dipoles X and Z orientation with respect to the elements of the beam are in complete misalignment.
Since the radials are not directly above any one of the elements on the beam, radiation taking place on the radials is either physically ahead or behind any of the elements on the beam. This is what causes the pattern of the beam to become tilted and skewed. Since the dipole / radial is not at the same height as the beam and inline with its reflector and directors, any currents on the radials will also be bi-directional on TX and RX.
While every installation will have variables that can alter the degree of parasitic coupling, why risk it at all since it's not necessary? Whenever possible we should avoid placing another resonant antenna of the same polarization and within a wavelength of each other. Unfortunately the horizontal radials of the groundplane place it right in this category as far as the beam is concerned.
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Well guys Shockwave is exactly right, and this is why we might have missed completely what he was getting at.
Here is my model (Yagi 2- SP500 above 108") in free space, and it does show what is said.
I realize you guys might not get much out of Free Space models and complain sometimes, but it requires a different perspective to appreciate how they can be important. They do demonstrate much better how antennas or affected by their construction, the mast, and their location relative to Earth like SW said, and what is close by if any other thing in included in the model.
By removing these losses from the model, we see a much better picture of the true nature of the design. Of course the Earth and the stuff on the surface changes what happens, but we do need to consider FS ideas before we build the design, and then later we can test in the real world. Does this make any sense for why we need FS models in modeling?
View attachment Shockwave's idea in free space.pdf
Wire Dawg, I can't specifically answer your concerns, but there would be some differences. However, I think the basic results would be about the same. One thing going on here might be cancellation effects that have not been discussed, and that could come into play with the model over real Earth, but that is just a guess on my part.
My antenna experiment years ago with a Starduster that have radials about 7'+ hanging down below the hub, was mounted on a 9' mast above my 4 element horizontal yagi...and I detected NO differences just using my radio. When we put our antennas over real Earth...things are a bit different it seems.
Good luck and keep us posted.
I forgot to add the model of the Yagi 2 by itself in Free Space and you can see for sure the GP is the cause of the pattern difference.
View attachment SW's idea without the GP antenna..pdf
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