An antenna experiment that Freecell once told me about.

[Marconi]

Some of you may have heard a similar story before from FC.

The other day I was reading some old radio notes I use to keep in my notebook. I read an entry where I was talking to Freecell on the phone way back when. He told me about his experiences hanging a Starduster upside down in his hay barn where he lived on the side of a mountain about 300' above the surroundings to his West. He said the barn had three decks and was over 50' feet high.

I asked why he would do such a thing, and he told me so that he could put his signal off the mountain side, right down onto the cities below and to his West.

He said his upside down ground plane put the RF signal a few degrees below the horizontal and it gave him an advantage from his 300' foot higher location. He also said it raised his signal up the mountain, and he experienced much improved DX signals from the East as well.

I told him I though he was full of it, but I couldn't prove anything.

Does that make any sense to you guys?

Would his signal do like he said, with the antenna upside down?

View attachment Freecell's upside down idea.pdf

 

[gamegetter]

interesting, I remember you mentioning this once before on a thread,,,i think.

did the software give you total field and elevation plot?

I would think cmc's would be excessive.

 

[hookedon6]

This inverted-mounting technique is commonplace in cities such as Johannesburg, Perth, Invercargill, Lima, and Buenos Aires.


I always had to stand on my head to work those guys, anyway.

 

[hookedon6]

ok,......... real answer:

many a repeater operators have mounted ground plane antennas upside down on mountaintop radio sites so that the main lobe is now aimed BELOW the horizon, where the desired coverage area is. In cases where the coverage area is near the base of the mountain, it often works MUCH better than a higher gain antenna mounted right side up.

 

[tecnicoloco]

The Sigma 4 FM type antenna (Dominator) from Shockwave has a downtill radiation as he claims

FMBroadcastAntenna.com | FM Broadcast Antenna's

http://www.worldwidedx.com/cb-antennas/26885-sigma-iv-fm-broadcast.html

 

[Marconi]

interesting, I remember you mentioning this once before on a thread,,,i think.

did the software give you total field and elevation plot?

I would think cmc's would be excessive.

Gamegetter, I'm not sure, but I had the same thought. Like I said earlier, back then I had no way to prove anything except for words, but I told him I didn't think that is the way a SD would act...if it was placed upside down.

I'm working on two models, one of a SD'r and the other with it upside down like I showed earlier. I'll post when I finish.

GG, these models don't have a radiator or coax, so I can't tell about CMC, but I'll check that out. I recall FC saying he hung the antenna from a mast that was attached to a rafter that put the antenna up about 50' feet high.

 

[Marconi]

Here is Freecell's upside down SD'r vs regular SD'r.

These models are a 36' feet with no mast of feed line. I used the same model details for both models.

View attachment FC's upside down SD'r.pdf

According to these models, FC was wrong, there is no downward tilt noted. My assumptions were based on the SD looking to me like it is a 1/2 wave radiator and producing a 1/2 wave pattern...where the bottom and the top look the same, and flipping over should also be the same.

 

[Marconi]

The Sigma 4 FM type antenna (Dominator) from Shockwave has a downtill radiation as he claims

FMBroadcastAntenna.com | FM Broadcast Antenna's

http://www.worldwidedx.com/cb-antennas/26885-sigma-iv-fm-broadcast.html

I think SW has modeled his Dominator, and he can probably show us the down tilt his antenna's radiating pattern. It is just my opinion, but I would imagine if the Dominator does show a down tilt...it would be indicated with a Free Space model and not over Real Earth.

FreeCell also use to also talk about doing something with an AstroPlane to get it to tilt down. However, I have manipulated my AP model in all different kinds of way, but I don't think it ever showed a effective radiating angle below the horizon.

I forgot to add an overlay of the two models above.

View attachment FC's idea with overlay.pdf

 

[Shockwave]

The Sigma design actually has a free space peak gain between 1 and 2 degrees above horizontal 0 degrees. We have been working on ways to stack a pair, add null fill and beam tilt. One method does indeed involve inverting the lower antenna to hang it upside-down. We may also begin recommending stations using a single antenna at extreme elevations, side mount the antenna upside-down.

Years ago when I worked for a Motorola authorized service shop and low band FM was popular, we also hung ground plane antennas upside-down. One installation I recall was for the Department of Public Works in a nearby city. They were on 45 MHz. As the peaks of solar cycle 22 approached, the complaint was "We keep hearing Alabama coming over the Connecticut DPW frequency.

One of the Motorola engineers advised us to invert the ground plane and reduce the height that it was mounted on the tower, down from about 120 to about 80 feet. I'm not sure if the height reduction was an attempt to put Alabama in one of the antennas nulls but I do know the antenna inversion was an attempt to reduce skywave activity and it worked fairly well. While Alabama could still be heard, the attenuation to their signal was noticeably more then the local ones.

This is what should be expected of an unbalanced vertical antenna. Their counterpoise tends to push the signal slightly upwards. If you invert the antenna, the opposite affect is true, the signal is of a slight downward tilt. What works best depends on your desired coverage area and the location the antenna is used in.

This also suggests that FreeCell's claims of improved downward coverage from his mountaintop location while experiencing simultaneous improved skywave coverage is not likely or to be expected. You can't claim you've improved gain below the horizon and in the good skywave range about 15 degrees above the horizon at the same time.

It is more likely that the simple change in height cause by inverting the antenna has favorably changed one of the antennas secondary lobes to begin targeting skywave. When the antenna was right side up, there was probably a deep null around 15 degrees due to the specific height it was mounted at. This is one of the reasons Tom says TOA alone is useless. Because you need to know the beamwidth and where the secondary lobes and nulls will appear to fully understand the coverage area.

 

[Moleculo]

A lot of public service / police / fire repeater antennas around here are hung upside down for the reasons already explained. It's a pretty common practice.

 

[Shockwave]

Thanks for posting the radiation plot Marconi. Unfortunately it leaves me with more questions then answers on this inversion idea. I do know the Sigma design shows a slight upward tilt when modeled in free space. This suggests that the antenna itself is influencing that tilt due to its unbalanced design. While I don't expect to see the same angles in free space as we do over real earth, I would expect to see some sign of this tilt in both. Since you already have the same antenna modeled both inverted and normal, could you click on the free space option and run those plots again to see if this antenna displays any tilt at all?

 

[W5LZ]

As the modeling program shows, there is no significant difference in radiation patterns. That would indicate that it wouldn't make any difference how the thing was mounted, it would produce the same radiation pattern. The difference between 10 degrees and 30 degrees? I'd bet that had more to do with the height above effective ground than the thing being right side up or up side down.
Try this. Model the same antenna mounted at 90 degrees, or horizontal. The radiation pattern will be very similar to a typical dipole antenna.
What's all that prove? That those radials DO radiate! Same amount of radiation from under those radials as off the pointy end of the antenna.
So how would you run the feed line to minimize CMCs? Same way you would if the thing was right side up. Run it vertically for as far as possible then bend it over to get to the radio. Would there BE any CMCs? Probably. 'Cure' them the same way you would for any CMCs.
- 'Doc

 

[Marconi]

Thanks for posting the radiation plot Marconi. Unfortunately it leaves me with more questions then answers on this inversion idea. I do know the Sigma design shows a slight upward tilt when modeled in free space. This suggests that the antenna itself is influencing that tilt due to its unbalanced design. While I don't expect to see the same angles in free space as we do over real earth, I would expect to see some sign of this tilt in both. Since you already have the same antenna modeled both inverted and normal, could you click on the free space option and run those plots again to see if this antenna displays any tilt at all?

My Sigma model does show a little upward tilt of about 15* degrees in free space, but 0* degree is only slightly less in gain, so I don't see that as an issue, but I do agree with your thinking it is probably due to the asymmetry in design. The SD'r is not perfect, but its pattern is very similar to a 1/2 wave dipole, and the bottom half of the antenna is approximately the same as the top half, thus the pattern shows maximum gain at 0* degrees in free space. Over real Earth or in free space the upside down model of the SD'r shows no downward tilt of.

I just don't see what all these upside down users see turning their antennas upside down, whether a 1/4 wave or longer. I would have to see some other proof. Just because somebody says something, does not make it true in my book.

View attachment FC's free space models of SD'r.pdf

FreeCell and I were talking about a Starduster, and not a Sigma or a 5/8 wave, but I did turn a 5/8 wave upside down to see what happened to it, and the nice low pattern noted over real Earth went to heck in a hand basket when upside down. In free space I do see some tilt down, so I guess the tendency to tilt down is there for an asymmetrical antennas in free space, but it looks like Mother Earth has something else to say about that over real Earth. I would expect the Sigma to show about the same results.

View attachment FC's upside down idea for a .625 wave.pdf

This all gets a bit confusing for me switching back and forth with these models, so I hope I have correctly related here what I see and think.

 

[Marconi]

As the modeling program shows, there is no significant difference in radiation patterns. That would indicate that it wouldn't make any difference how the thing was mounted, it would produce the same radiation pattern. The difference between 10 degrees and 30 degrees? I'd bet that had more to do with the height above effective ground than the thing being right side up or up side down.
Try this. Model the same antenna mounted at 90 degrees, or horizontal. The radiation pattern will be very similar to a typical dipole antenna.
What's all that prove? That those radials DO radiate! Same amount of radiation from under those radials as off the pointy end of the antenna.
So how would you run the feed line to minimize CMCs? Same way you would if the thing was right side up. Run it vertically for as far as possible then bend it over to get to the radio. Would there BE any CMCs? Probably. 'Cure' them the same way you would for any CMCs.
- 'Doc

Yep 'Doc I did .25, .50, .625, .64 antennas laying on their sides the other day in another thread maybe, and in that position, with each the same distance above Earth, you can hardly tell enough difference to really matter.

My point was that except for the height of the current nodes being different due to the antenna design, these antennas probably all produce about the same gain and angle...when the height advantage/disadvantage is removed from the test.

 

[W5LZ]

And just for grins.
That radiation angle isn't determined by the symmetry of the antenna so much as by it's height above ground. There's no 'ground in free space so different radiation angle naturally. The 'shape' of an antenna can have an affect on radiation angles but not as much as 'dirt' does. So, use that dirt!
- 'Doc