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LPFM antenna strategy

J

JimmyJames

Guest
I'm reading tons of different opinions on this and thought I'd see what people here had to say about the ideal antenna config for an LPFM. I want to get the max coverage for 100 watts, and wanted to know what to expect and if there were any particular (but legal) tricks I could use to make the most of it.
 
Just be wary of antenna suppliers advertising high power gains for FM transmit antennas that consist of only one bay, or element.

LPFM stations are licensed on their ERP, so if you are using a v-pol or h-pol antenna with supposedly high gain (more than 2.15 dBi, or 0 dBd) for a single element then you could wind up radiating less real ERP than you think you are.

An unrealistically high gain figure for the antenna.means that less transmitter power is required for a given ERP. If that gain doesn't exist except in ad copy, then the stations ERP will be less than the licensed value,

It also might lead to the purchase of a transmitter that isn't rated for enough output power to produce the licensed ERP when using the true (lower) gain of that antenna.

//
 
...I have two LPFM's I set up as Engineer.. I used NiCOM for a good exciter/amp and their two band processor on one and an old Optimod-8100 for the other one.. I have one bay on one and two bay (circular polarized, on both) on the other... I have been very impressed.. Station "A" is a two-bay with better fringe coverage at the horizion (considering inversion co-channel problems in the early AM drive, but we are youth and alternative, with about 60% of our listening coming on the internet side)... I cuts through well at a half wave length and a .7 gain... It's 58 watts at 130 feet... Been smokin' for three plus years... Station "B" has been around a little less of time, but with one bay and a little hotter NiCOM exciter/amp, it equals the two bay on Station "A".. I do hear a little more interference in the 50 to 40 dbu, compared to the two bay setup.. But, the local 50 dbu and better signal covers the local school district and the local university's local area well.... With a Behringer mini-board as the 'eq' and pre-amp mixer from the computer/Zararadio source to exciter input and set at 0 to +1 on the processor from NiCOM, it sounds like a real station! The second station takes a dedicated web-stream from 9 miles out and I need to get the kid running the day to day operation to tweek the Window's Media Player feeding the Optimod-8100 to pull down low-mids and up slightly on the mid-highs...Also, I heard a grounding hum that can be knocked out by the Symetrix AGC/Leveler gate knob.. I do not have an AGC on the other station, though I'd love to put one in to level out some hotter material on the Zara software (the Zara built in AGC is light).... Oh, the one bay LPFM is 93 watts at 31 meters and has a one bay gain of .49 .....
 
KXFM LP Lancaster, California uses a 250 watt transmitter from Crown to feed a single bay on a large faced cell tower. The station can be herd 45 mile out, from Gorman/Frazier Park Ca to the west to Mountain High on the east side, and Mojave to the north. The tower is located at Ave A & HWY 14 freeway in the Antelope Valley.

Steve
www.knjoradio.com
 
In my experience, these antennas don't help much with building penetration and there's lots of holes. Don't know if I'd use one for LPFM, but I'm curious how many do.
 
Really stuff for amateur part 15 type of test operations.. It does not draw much attention for pirates that want to cover their campus without an obvious horizontal, vertical or one bay circular polarized antenna on a pole... I think, per dollar, NiCOM has met my expectations, above and beyond.. When one unit did not power up right, they made good within four days! I was only set back four days to ship out and get a new one back! WOW! It gave me more time to get the studio more in shape... They have a full band element antenna that will not need retuning if you switch frequencies.... It's about eight hundred a bay element...
 
What's the verdict on something like this:

http://www.progressive-concepts.com/info/item.html?id=47

Seems sort of too affordable to be true, is that so?
Click to expand...

I have used several of the Comets for translators. They are very narrow band, so you pretty much have to tune them after they are installed on the tower (a real PITA).
In testing, the loading coil gets noticeably warm with only 15 watts into the antenna.
The price is cheap, and that is what you get.

Want the most bang for your station? Then use a single bay wideband CP antenna.
You will get the best building penetration, and the least multipath. (but the highest electric bill)
I would highly recommend the Bext TFC-2k. http://www.bext.com/pdf/P046-TFC2K.pdf
I own translators in some major markets where I cover 500k people with 75 watts, and that is all I buy for them.

-3.4 dbd gain means you get 100 watts ERP with ~218 watts at the input to the antenna.
Its a full band wide, CP antenna (no tuning issues, even if you change frequency), built like a rock, radiates very well, and can be had for around $750 new.

Finally, calculate your TPO correctly (including coax and connector losses) to make sure you get the most power you should have.
 
Cheap...even a caveman can assemble it.

Univ. of AZ had one on their LPFM (Student radio) It kicked out 2 sq. miles on a good day at 20 watts. ;D
 
Univ. of AZ had one on their LPFM (Student radio) It kicked out 2 sq. miles on a good day at 20 watts.
Click to expand...

Useable antenna at minimum expense...yes.

To maximize coverage for 100 watts ERP...its a poor strategy for that.
 
The Comet antenna is used widely by undocumented broadcasters worldwide, one guy in the Antelope Valley used a Comet with a 30 watt input to blanket an area of 300,000. I have a 1/4 wave version of the Comet; the antenna produces a clover leaf like pattern with three deep nulls, my dipole blows it away with a 1/10th of the power.

The Comet is good to have around for a back up antenna!

Steve
www.radiobrandy.com
 
The Comet is a good antenna for low power vertical only broadcasting. It gives you some gain without weighing a lot.

But it is not a good choice for LPFM in the United States. Portable radios use vertical whips. But many car radios now use horizontal in the windshield antennas and home stereos mostly use horizontal as well.

Tests have shown that vertical only FM will cause a 20 DB loss in the horizontal plane. In other words, 100 watts
of vertical only LPFM will get out like 1 watt on a horizontal antenna.

As far as antennas for FM go, ERI and Harris are the best. But they also cost the most. Get what you pay for.
 
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