Antenna Height vs Gain

[ButchKidd]

Can you guys give me your general thoughts on this?  I know there is no right answer.

I've finally built a circular polarized antenna to try to get a more consistent signal no matter what type of receiver.  My original idea was to build a second CP antenna and feed it in phase a wavelength below the first.  Now I'm having second thoughts on the two-bay idea.

I'm in a rural area and my existing ground plane antenna is nearly 100 feet above the terrain.  It doesn't quite clear the treeline, but it's as high as I'm likely to get without a real tower, which isn't going to happen.  It's a good transmitter site, but despite some pretty cavalier power levels, I just can't get the signal to my house.  I think I'm terrain-limited rather than power limited.

Here's my thought.  I'm not really comfortable using high power long-term.  It seems like asking for trouble.  I'd like to get the most out of a lower output power, but if adding another CP antenna will just lower the center of radiation by one half wavelength, would you expect to get any additional range out of the higher gain?

All things considered, I can just run more power to a single antenna at the same height.

[JimIO]

You don't say what power or range. If the antenna is above the terrain then a ground plane is not a good antenna to use. I would have started with a center fed half wave vertical. What design is the CP antenna?

[ButchKidd]

With 20 watts, I was getting 3-4 miles.  At 50 watts, about 6 or 7 miles.  I did some quick tests at 100 and 150 watts, but the range didn't improve.  RDS lasted much closer to the edge of the signal,  though.  There's a hill roughly 7 miles from the transmit site.  When I crest the top of that hill, I lose my station and most of the time  I can just make out the next nearest station on the same frequency 60-70 miles away just above the noise.  I live a couple miles past that hill.  Most of the town can hear the station just fine, but my neighborhood can't hear it at all.

I realize I'm already getting better range than a lot of licensed LPFM stations.  I guess that's because we're so far away from cities and the noise floor here is comparatively low.

JimIO, I agree with what you said about a ground plane antenna that is very high above the terrain.  I just didn't consider 100 ft to be very high when commercial antennas are 800 ft high.

The CP antenna I built is a turnstile, so it's pattern should be much more like a center-fed dipole.  Well, like two of them, but you know what I mean.  I just took the designs for 2m and weather satellite antennas, removed the reflectors, scaled them up, and turned it on it's side.  It seems to work well in the yard, but I won't really know how well it performs until I get it up in the air.

There are a couple churches kind of near here that have LPFM stations, and it looks like they're both using Jampro Penetrators or something similar.  I might try to build something like that in the future, but I understood how a turnstile worked.  I don't know what type of matching the Jampros use.

[JimIO]

Big stations on tall towers use beam tilt. Way beyond free radio.
It sounds like you are terrain limited. Where is your studio?
What about a second transmitter to cover your neighborhood?

[ButchKidd]

Thanks for the help, JimIO.

I record my shows at home and upload them to the computer at the transmitter site via WiFi when I'm nearby.  There's no internet service at the transmitter site or I'd stream it.

A second transmitter is probably the most practical solution.  I've been reluctant to set up a pirate transmitter at my house, since I thought all the existing antennas would give it away.  I still have the cheap Chinese transmitter I started with years ago.  I could probably find a friend who would be willing to host it as long as I keep the power really low.  I'd like to find a way to get the two broadcasts in sync, but I guess that's more complicated than it's worth.

[ThaDood]

Well, not me, but someone whom kindda' resembled me, did well with 25mW, 40ft up a fur tree, on a home brew copper, 1.1:1VSWR tuned 1/4-wave GND Plane. It was heard over 5 miles away in one direction. Then, 400mW was heard almost 10 miles away in the next state. (Damn, that station is sadly missed.) And, even got a letter writing back to a Radio Garbanzo broadcast from a town over 70 miles away. Even Fearless Fred said, "Ya never know!". (Temp inversion that night?) CP wasn't done since it was just seen as wasteful. 1/2 the signal either goes straight up, or straight down. (OK for worms or ET.) FM is no longer done, due to the huge FM witch hunt the Federal Corporate Coddlers are so hard on about. Any chance to get your antenna in a tree, just above the tree? I remember a station, in fairly flat terrain, used just 3W up a 60ft pine tree on one of the custom made 1/4-wave GND Plane. It MAX'ed out to 19 miles in one direction! Huh... Wonder what ever happened to that dude whom resembled me? 

[Josh]

A vertical calculated for beam tilt would help in the range dept, or a vertical dipole(s). The big boys use cp because they're trying to fill concrete canyons with flutter - free fm stereo as well as horizontal fringe antennas at the same time as vertical car antennas.

[OgreVorbis]

Can you guys give me your general thoughts on this?  I know there is no right answer.

I've finally built a circular polarized antenna to try to get a more consistent signal no matter what type of receiver.  My original idea was to build a second CP antenna and feed it in phase a wavelength below the first.  Now I'm having second thoughts on the two-bay idea.

I'm in a rural area and my existing ground plane antenna is nearly 100 feet above the terrain.  It doesn't quite clear the treeline, but it's as high as I'm likely to get without a real tower, which isn't going to happen.  It's a good transmitter site, but despite some pretty cavalier power levels, I just can't get the signal to my house.  I think I'm terrain-limited rather than power limited.

Here's my thought.  I'm not really comfortable using high power long-term.  It seems like asking for trouble.  I'd like to get the most out of a lower output power, but if adding another CP antenna will just lower the center of radiation by one half wavelength, would you expect to get any additional range out of the higher gain?

All things considered, I can just run more power to a single antenna at the same height.

I have some decent experience with this although I've never tried a CP antenna. I've tried ground plane antennas, dipoles, and the "dominator" antenna (which is a 5/8 wave with interesting matching). Anyway, in a hilly location, I've actually found the ground plane type antennas work better at a lower height than anything with higher gain. The lack of gain almost compensates for low height (I'm considering 100ft - low). It shoots the signal up more. Yes, some of it will be lost in space, but on the low end of the FM band, there's some bending that can occur and the GP will cause the signal to go over hills a bit better.

With the amount of power you're using, you should be getting farther than that. My guess is the transmitter site is just in a bad location. Something I've thought about doing, was putting a solar powered TX on a hill nearby with a directional wifi dish for streaming audio and controlling the transmitter. You could get away with only 15W on a hill and go much farther than 150W on the ground. Height is pretty much everything, not power. I did a test with a 30W transmitter on a mountain that I lugged up with a car battery (wasn't easy). The signal went about 30 miles.

Another thing to consider if you just want to make it from the site into town is a yagi.

One more thing - Don't use stereo. Give mono a try. When you're really close to the TX, it won't sound as good, but I came to find that with my setup, using mono made the signal much clearer longer distances away. And at those distances, nobody is going to be hearing the advantage of stereo because receivers default to mono with low signal level anyway.

Are you getting multipath distortion? When you drive listening to the signal, does it pop in and out. Sounds  pretty clear, but with bursts of static. If yes, use mono or SSB stereo mode instead of normal stereo.

[ButchKidd]

I put the turnstile antenna up yesterday, and even with the -3dB from the CP antenna, I'm already getting better results.  It's not vastly different, but certainly better.  That makes me think it is at least partly multipath issues.  Since the parts for the turnstile weren't expensive, I'm thinking of stacking two again.  If it doesn't help, I can always take the second one back down.

My wife's car has one of those horizontal trace antennas on the glass, and I can pick my station up all over town with it now.  I realize that's cross-polarization for the old ground plane, but that seems to be what many cars have like it or  not.  I notice much less picket fencing when I drive around town between steel and masonry buildings, even with a vertical receive antenna.

And I can receive it at home if I use my attic mounted TV log periodic. It's too weak for stereo and RDS, but I can hear it.  Certainly not strong enough for a casual listener to stop on it, though.

I agree that I think I should be getting way more range for any given power level.  Do you think I could be under-modulating?  I don't have an accurate way to check the deviation like an oscilloscope or FM analyzer, so I've tried to look at the bandwidth on an SDR dongle and keep it about as wide as the other stations.  The nearby stations sure seem to take up more than 150 KHz on my waterfall, so maybe I've been too conservative.

[JimIO]

Keeping it short, using a V tx ant. and H rx ant. you will be up to -20db.
As for modulation, crank it up until you are as loud as other stations.

[Josh]

In a perfect world you'd have the town/area you wish to cover between you and the tx site, then you just use a beam to do the heavy lifting. Also, a beam on your end would help.

When I was in ems and in the sticks, often driving a bit below the crest of a hill (on the opposite side of the hill from the repeater) would allow me to knife edge into the repeater, uhf is funny stuff.

[ButchKidd]

I put up the second turnstile antenna Friday, and fed both of them in phase with two 3/4 wave lengths of RG-11 mounted a whole wavelength apart.   I did several tests, for both range of the detectable signal and how well it covered the local area as far as signal strength, reliability while driving, fading, picket-fencing/multipath, etc. at 1, 2, 5, 10, and 15 watts before and after adding the second bay.

For my calculations, I assumed I'm getting somewhere close to the theoretical 3 db gain from the 2 bays,  which brings the total gain of the array up to 0 db, since a single CP dipole is -3 db.  In reality, I know it's not that high with four additional mechanical connections for the tee and second antenna as well.  The additional cable loss is probably negligible.

As you would expect, on hilltops with a clear-ish line of sight to the antenna, I got a little more range with the 2-bay array for each power level.  In fact, with 15 watts I was able to "hear" the station on extremely distant hilltops, although it certainly wasn't what I would call "listenable".  Using a communications receiver (fancy scanner) with a mag mount antenna I got the expected rise in signal strength of around half an S-unit (3 db) for a given power with a reasonably clear line of sight.

But the surprising part was, as OgreVorbis suggested, in town I actually got worse performance between brick/stone buildings using the 2-bay array.  I don't usually think of a 3 db difference as very big, but I understand that gain has to come from somewhere.  I just didn't expect a 3 db difference at the horizon to result in a 1-2 S-unit (6-12 db) difference in some places in town.

One other thing I wasn't taking into account was RF horizon.  At 100 ft the curvature of the earth limits line of sight to 12 miles with an RF horizon of about 14 miles without any obstructions like trees or hills.  So without help from favorable propagation, not matter how much power I use, I won't get further than that without finding a new transmitter site.

It's been a fun experiment, and I'll probably leave the 2-bay array in place for a while, but it looks like I'm just going to go back to a single CP crossed dipole, as that seems to be the best performer for me.  And now that I have a second antenna, I might be able to set up another transmitter on my side of town for my own "translator".

The beam idea JimIO suggested is a good one, and I might experiment with some kind of reflector behind the existing CP antenna.  For the time being, I want to stick to circular polarization, because I understand cross-polarization losses when using linear polarization.  That's why I started experimenting with circular polarization in the first place.

Thanks for all your help and ideas.  If I decide to take on a CP beam, I will start a new thread focused on the build.

[JimIO]

Was Josh who said beam. But if you're thinking of going there how about a helical.
Just take a plan for a WIFI one and scale it up. I would love to see somebody do that.
That's a real CP antenna.
 

[ButchKidd]

An end-fire helical for the FM broadcast band would be a beast. The ones I've seen for MIL SATCOM are impressive, and this would be at least three times as large.  I might have to save that particular design for when I retire.

[redhat]

A few thoughts here;

Everyone's situation is different, but the same rules apply.  If you have terrain shielding, nothing is going to get you to a receiver on the other side of a mountain....nothing outside of putting the transmitter on said mountain.

Without a clear understanding of the radiation pattern of the antenna your kinda shooting in the dark.  I've had good luck with ring and stub antennas for CP over the years as they are light, fairly easy to construct, but a bastard to tune.  Being somewhat horizontally polarized mutual coupling will change their VSWR response when coupled as an array.

If your primary audience is close in, use fewer bays and higher TX power.  For more distant listeners, use more bays and lower power.  You can play games with beam tilt by making one of the phasing lines longer than the other, but if terrain is your problem, nothing will fix that.

There is no magic antenna.  All are a compromise suited for different situations.

For the record, helicals are impractical at FM frequencies.  I was toying with the idea at one time, but to get any kind of gain at 100 MHz the thing had to be about 2' around and 20 feet long.  I had better luck with a co-linear jpole with phasing loops.

As for modulation, either get one of the pira analyzers, or use and FM tuner with its baseband brought out to a peak responding meter, or a scope.  Then compare your modulation to the other stations.  DO NOT try and make yourself as loud as heavily processed FM stations, as most likely your processing is not capable of such density and you will wind up over deviating.  The SDR trick is a good work around for you in the meantime.  Also keep in mind that running subcarriers (RDS) will muttle your ability to discern a stations peak deviation.  FCC rules allow stations to run over 100% modulation by 1/2 the deviation of the principle subcarrier in use.  In other words, if a station is injecting 4% RDS, they are allowed to deviate at 102% peak modulation, or 76.5 KHz.

+-RH