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Messages - ButchKidd

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FM Free Radio / Re: Antenna Height vs Gain
« on: July 31, 2018, 1431 UTC »
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.

FM Free Radio / Re: Antenna Height vs Gain
« on: July 24, 2018, 0132 UTC »
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.

FM Free Radio / Re: Antenna Height vs Gain
« on: July 20, 2018, 0014 UTC »
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.

FM Free Radio / Re: Antenna Height vs Gain
« on: July 19, 2018, 1222 UTC »
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.

FM Free Radio / Antenna Height vs Gain
« on: July 19, 2018, 0200 UTC »
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.

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