Loggings > FM Free Radio

Antenna Height vs Gain

<< < (3/3)

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

Navigation

[0] Message Index

[*] Previous page