Thoughts on mounting 2m/70cm antenna upside down?

[AA4BK]

Hi everybody,

I'm in a bad location for reaching most of the local repeaters. I am in a very hostile radio environment with a lot of noise, and a lot of steel and concrete around blocking signal, with a bunch of trees out the only side of the building I have external access. I also don't have a way to mount an antenna outside (other than hanging it in my window), so that limits things even further. In addition, in one of my two external windows I have a J-pole hanging for my SDS scanners.

I have a 2m/70cm magnet-mount, mounted upside-down off the ceiling (because that's the only place I can really put it). My question is, does it matter if it's inverted? I'm able to hit one repeater site (and only one) about 23 miles away using around 2.5 watts on 2-meter, and one a few miles away on 70cm. I'm working on improving the antenna (addition of a ground plane, etc.) but I just wanted to get opinions on whether it matters if it's upside-down or not.

My thinking is it shouldn't matter too much. It's vertically-polarized so shouldn't make a difference from that point of view. On the emissions charts I know that in VHF most the energy is radiated horizontally off the active element, so that's good as well. Now, on UHF I have a few more concerns, though. UHF, of course, has several lobes because the UHF is operating using a harmonic, so I have a few lobes at various skyward angles, with still of course a large lobe emitting horizontally.

Except........because the antenna is inverted, the skyward lobes are actually now groundward lobes. So I'm going to lose a lot of that energy probably. So that's not good.

Also for clarity: That J-pole I mentioned is not for transmitting, it's just for the scanners.

Anyway I'm interested in thoughts anyone has. If the inversion is okay, I'll just construct a ground plane "under" (actually over) the antenna and call it good. Otherwise I'll need to re-evaluate my whole antenna situation and look for alternatives again.

 

[Ubbe]

If it's a "clean" 1/4 wave antenna at 144MHz it will works as a 3/4 wave at 3 times the frequency at 433MHz. But as I understand you guys in US have a different 70cm HAM band to use so the match might not be perfect. But I suspect the antenna have some gain and then use other wavelengths using phase coils and perhaps a loading coil to make the antenna work as perhaps 5/8 at 2M and several elements stacked at a better tuned 70cms band? What model is it? Anyhow when having it indoor it doesn't matter if its inverted. You have so much local interference in the house that it takes over any issues with ground noise or noise coming from the sun depending of where sidelobs end up.

/Ubbe

 

[trentbob]

Just curious, can you open up the use of the window that has the non-transmitting J Pole that you use for your scanners? How's your performance on that anyway?

Why not use a good quality antenna like a Larson tri-band on a pie pan or cookie sheet for the scanner and try to put a ham antenna in that window?

 

[Ubbe]

A J-pole are narrow banded, not well suited for multiband use and it's very sensitive to nearby metal objects. You will have to isolate it from a metal mast and decouple the coax as it is a balanced antenna and the coax are unbalanced. You usually have a metal frame around a window and the tip of a j-pole, that are very high impedance, comes very close to the metal that ruins the antennas performance. But any metal in the air working as an antenna when placed in a window will probably work better than the telescope one at the back of the scanner or a rubber ducky on the portable.

/Ubbe

 

[murphcc1]

I have a Larsen Tri Band hanging upside down in my window, the magnet connects to the metal channel corner bead.
I have no issues with it!

 

[Lauri-CoyoteFrostbite]

Another opinion----

I don't think it matters a great deal the orientation of your antenna(s), bfk. With all the other factors you mention, the walls and nearby metal structures adding to the mix --the polarization, reflections etc. make modelling an antenna impossible. If I understand correctly, you are interested in just two repeaters ?
If that is the case, I'd just move your vertical(s) about, trying different places within your building, looking for the 'sweet spots' that get you into the machines the best--- be these spots in windows or against some wall etc., inverting them or even horizontal.

For what its worth, I have inverted remote mountain top antennas to take advantage of the inverted lobe--- there is such a thing as too much height to an antenna --and all your signal will go sailing off over the target'd area and into space.

 

[merlin]

Doesn't matter if the antenna is pointing up or down,
down, it needs a standoff from any metal support structure.

 

[dsmiley]

So long as it doesn't collect water

 

[AA4BK]

Hi everybody! Thank you all for the many thoughtful responses. I wanted to follow up and answer a few questions and comments I saw come in:

What model is it? It's this one: https://www.amazon.com/dp/B095WR7368 and yes it does use coils, although I don't know the internal architecture to know exactly how the coils are used. But it definitely looks to me like a mag-mount J-pole. I'm new at this though and only just got my ticket so I'm still learning! I don't have an SWR meter yet but all the tests I've seen show respectably low (very close to 1:1, maybe 1.1:1) on VHF, and not much higher on 440 although it's of course using 3rd harmonic for 440.

Just curious, can you open up the use of the window that has the non-transmitting J Pole that you use for your scanners? How's your performance on that anyway? It's tough because I'm in a very harsh radio environment here. It's a medium-rise apartment complex, quite a large building and I'm near the center although I am at least on an outside wall with two windows facing out. I'm about halfway up the building. So I have RFI all around me, made even worse by the fact that I'm a computer guy and have a number of systems running, multiple wireless networks, cables running every which way, so there's a lot going on. I have the two windows facing the south exposure, and the J-Pole I normally leave on my SDS200 is up in the upper right corner of the right window. Down at the bottom left window I have a little HDTV panel antenna for my Tablo so I can record NASCAR races. I may try to move that over to the right window at the bottom to free up the entire left window for my mag mount (or whatever I end up using) to see if I can get it closer to the outside. But the problem there is if I put the antenna right in the window itself, I think it's going to be too close to some structural metal likely to be in the wall, and affect the electromagnetic characteristics of the antenna through coupling. So I'm reluctant to do that.

Why not use a good quality antenna like a Larson tri-band on a pie pan or cookie sheet for the scanner and try to put a ham antenna in that window? That's an excellent idea and I might actually try that. I'll have to think about a way to mount a cookie sheet horizontally about mid-way up my window, and that might actually be an option if I can get enough distance from anything metallic at or above the level of the cookie sheet in that location, while properly suspending it.

A J-pole are narrow banded, not well suited for multiband use and it's very sensitive to nearby metal objects. You will have to isolate it from a metal mast and decouple the coax as it is a balanced antenna and the coax are unbalanced. You usually have a metal frame around a window and the tip of a j-pole, that are very high impedance, comes very close to the metal that ruins the antennas performance. But any metal in the air working as an antenna when placed in a window will probably work better than the telescope one at the back of the scanner or a rubber ducky on the portable. That's very good info! In my case I'm only using the scanner J-pole for the SDS200, and the other antenna, the one that I've currently got mounted upside-down (but appears to me to also be a J-pole) is the one I'm using for my ham radio. Strangely, the scanner J-pole does appear to be doing a terrific job catching signals for my SDS200, although I listen primarily to 700 and 800 MHz on that unit, but sometimes other ranges. In all cases, the performance of the SDS200 went up significantly in my particular environment when I connected that window-mounted J-pole to the SDS200. I suppose it's possible the antenna is actually performing poorly and the scanner is just desensitized by all the RFI around me, so maybe connecting a lousy antenna is allowing it to work better. But it does, to my naive observation, appear to work very well for my SDS200. So I'm reluctant to change that configuration, but I would like to find a better solution for my ham radio.

If I understand correctly, you are interested in just two repeaters? No I'd actually like to be able to hit all the local repeaters, it's just that I only seem to be able to reliably (or really at all) hit two: One being a rather distant (about 23 miles away) VHF repeater, which I can hit with about two watts, and the other a very close DMR repeater on the 440 (although in this second case I'm assuming I can hit it because I don't have my Radio ID yet, I am waiting on a call sign change first, but this second site is VERY close to me so I'm certain I'll be able to hit that, probably on less than a watt).

For what its worth, I have inverted remote mountain top antennas to take advantage of the inverted lobe--- there is such a thing as too much height to an antenna--and all your signal will go sailing off over the target'd area and into space. Excellent point! And I've actually wondered about that as well. If the energy doesn't move in the direction of your intended recipient it doesn't much matter whether it goes into the ground or off into space, it's not helping you either way.

So right now, what I have is that mag-mount antenna mounted upside-down from the air vent, about 18 inches in from the edge of the wall, directly between the two windows, but I'm thinking about everything you all have told me and considering my options here. I have to consider impact on nearby equipment as well. For instance, I don't want to hammer my SDS200 with too much of the transmission field from the ham antenna, if I can help it, so I'm trying to maintain a little bit of distance between those two. Same for the HDTV antenna, because I don't want to blow the front-ends on either device. Also, if I get the ham antenna too close to any of my computer equipment, cables, or anything like that it of course induces hum on the audio. That's minor and I can live with it, but if I can avoid it I'd like to.

Another interesting thing related to all this is that I've noticed that if I stand in a particular location near the corner of the room, using just my ham radio's built-in rubber ducky antenna, I can fairly reliably hit a local UHF repeater. But only if I stand in exactly that spot, using the built-in antenna. This building has some odd characteristics though with radio waves, because for example on 162 MHz, the NOAA weather radio I have cannot receive anything unless I put it in a corner with the antenna leaned up against the corner of the wall. Then the NOAA weather radio station comes booming in. In another corner when I am able to reach the UHF repeater I mentioned, I am standing in a position very similar (though not the same corner). It's a completely different frequency range but I suspect there's something about the construction of the building that makes it difficult to get on the air. This, as well, motivates me to look for options for the windows, since those seem to be one of my best chances of getting an outside signal.

Thanks everybody for all the great information, and taking the time to write such well-thought-out responses. I'm definitely learning a lot. I'm still very new at this. Like I said I just got my ticket within the last 2-3 weeks or so, although I've been a scanner listener for decades. So I have a lot to learn for sure.

 

[prcguy]

As a coincidence I've been playing with a mag mount in my garage comparing a few antennas and looking at signal strengths. I moved the mag mount from a work bench to hanging upside down from a light fixture over the bench and signal strength dropped drastically. I made sure to pull the coax sideways and away from the mount for several feet to avoid interaction and it was still a huge hit in signal strength compared to sitting upright on the equivalent of a pie tin.

 

[AK9R]

But it definitely looks to me like a mag-mount J-pole.

It could be. It could also be a lot of smoke and mirrors.

My first reaction was that it's a slightly shortened 1/4 wave antenna for 2m and a 1/4 wave for 70cm with a common feed point.

 

[paulears]

This really bizarre. Antennas are not random magical items, they're designs based on physics. Antennas have polar patterns in the horizontal and vertical planes. All documents and quite well known. Some designs take the typical 'donut' shape of a ¼ wave. and morph it into new shapes either in the horizontal plane to focus the capture area to a narrow beam, and benefit from the gain this generates. Omni designs generate a bit of gain by lowering the radiation angle - so what was directed up, and potentially wasted is concentrated in a lower angle lobe. You can take a horizontal yago type antenna and turn it upside down and nothing changes, despite up being down and down now up. Do this with an antenna that is an omni and the beam that goes between horizontalk and say 45 degrees up, now goes horizontal and downwards. Great on the top of a mountain where up is pointless and down useful, but not much use anywhere else.

An antenna designed to work against a ground plane behaves very oddly upside down. A groundplane design, like a magmount used without the groundplane doesn't work like it was designed - the radiation pattern is warped and mishappen. This mishappen polar pattern in both horizontal and vertical planes can work for you or against, but it is luck. Just because something appears to work does not mean it does!

 

[ladn]

As a coincidence I've been playing with a mag mount in my garage comparing a few antennas and looking at signal strengths. I moved the mag mount from a work bench to hanging upside down from a light fixture over the bench and signal strength dropped drastically.

Not to mention the inverted PL/DPL tones !

 

[mmckenna]

It could be. It could also be a lot of smoke and mirrors.

My first reaction was that it's a slightly shortened 1/4 wave antenna for 2m and a 1/4 wave for 70cm with a common feed point.

Yeah, that isn't a J pole. That's similar to the Stico multiband antenna. Just a common feed point with two whips.

As for the mounting antennas upside down, it's going to depend entirely on the radiation pattern. Some antennas will launch lobes up above the horizon. Flip the antenna over, and it's now going to launch them below the horizon. Great for high sites, not so great for low level antenna inside a home.

Experimentation or at least studying the radiation patterns would tell you what you need to know. But, it is a hobby, so it doesn't really matter what anyone else thinks. If you can make it into the repeater, then it works.

 

[AA4BK]

Just to follow up on this, I tried some more experimentation tonight and, after numerous tries, I think I found a very specific position and orientation, mounted upside-down from the ceiling in exactly a certain spot, that allows me to work around 5 or 6 repeaters. So that's a large improvement, and I think that'll work. Any movement of the antenna in any direction from where it is seems to negatively impact my chances, as does orienting the antenna any differently.

So it's working now, at least for now, and hopefully continues. If it does give me more trouble I'll probably look for a new antenna. All I can think is that the combination of RFI and building construction is just horrible here as far as a radio signal environment, which is consistent with my experience on the scanners as well. I suspect they're using metal studs, and a lot of concrete, so that's probably not helping matters, But I think I can live with the current antenna configuration as long as it continues to perform as it is right now.

Thanks everybody for the feedback and help!

 

[mmckenna]

All I can think is that the combination of RFI and building construction is just horrible here as far as a radio signal environment, which is consistent with my experience on the scanners as well. I suspect they're using metal studs, and a lot of concrete, so that's probably not helping matters,

At work, two steel reinforced concrete buildings with energy efficient construction. I can stand on the top floor of one, look across and see the cell tower on top of the other, about 100 yards away. Cell phone won't work.

Building construction, especially in newer construction, has a LOT to do with radio performance.

Glad you found a sweet spot that works.

 

[Ubbe]

My first reaction was that it's a slightly shortened 1/4 wave antenna for 2m and a 1/4 wave for 70cm with a common feed point.

The diagram shows the very narrow 144MHz SWR as a standard 1/4 wave will, so that then have to be the short whip with the loading coil to shorten the length to half. It will be less gain than a full 1/4 wave's 2.14dBi.

The SWR diagram for the 70cm band looks like it matches the longer stacked antenna type that looks like a 5/8 colliniar on top of a shortened 1/4 wave, or something like that. 3.5dBi gain seems resonable.

/Ubbe

 

[wyShack]

In my experience, all antennas react to 'earth' to some extent so yes it will make a difference. How much would depend on a lot of factors. If you are using a handheld (so limiting RF exposure) and it works, i wouldn't worry. Most actual operation is a compromise and this sounds like one more. 73

 

[JMSchlus]

Hi everybody,

I'm in a bad location for reaching most of the local repeaters. I am in a very hostile radio environment with a lot of noise, and a lot of steel and concrete around blocking signal, with a bunch of trees out the only side of the building I have external access. I also don't have a way to mount an antenna outside (other than hanging it in my window), so that limits things even further. In addition, in one of my two external windows I have a J-pole hanging for my SDS scanners.

I have a 2m/70cm magnet-mount, mounted upside-down off the ceiling (because that's the only place I can really put it). My question is, does it matter if it's inverted? I'm able to hit one repeater site (and only one) about 23 miles away using around 2.5 watts on 2-meter, and one a few miles away on 70cm. I'm working on improving the antenna (addition of a ground plane, etc.) but I just wanted to get opinions on whether it matters if it's upside-down or not.

My thinking is it shouldn't matter too much. It's vertically-polarized so shouldn't make a difference from that point of view. On the emissions charts I know that in VHF most the energy is radiated horizontally off the active element, so that's good as well. Now, on UHF I have a few more concerns, though. UHF, of course, has several lobes because the UHF is operating using a harmonic, so I have a few lobes at various skyward angles, with still of course a large lobe emitting horizontally.

Except........because the antenna is inverted, the skyward lobes are actually now groundward lobes. So I'm going to lose a lot of that energy probably. So that's not good.

Also for clarity: That J-pole I mentioned is not for transmitting, it's just for the scanners.

Anyway I'm interested in thoughts anyone has. If the inversion is okay, I'll just construct a ground plane "under" (actually over) the antenna and call it good. Otherwise I'll need to re-evaluate my whole antenna situation and look for alternatives again.

Many commercial antennas are mounted upside down on side arm tower mounts. They seem to suffer no problems as long as they are properly protected from water infiltration into the connections and base. All antennas are compromises depending on what limitations your situation requires. Try it out and let us know how it works out!

 

[paulears]

Are they? Never seen any upside down ones here?