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TELECOM FM Broadcast Antenna's

Bill Betten

Bill Betten

I have a client that is replacing some equipment at their LPFM and the antenna is of concern. I have searched on the net and can't seem to find much on the antenna system, this is from the original application.
"VERTICAL COVERAGE IS PROVIDED BY A SINGLE TELECOM MDR1 OMNI FM FOLDED DIPOLE. POWER GAIN IS 1.4X, OR 1.5 DB OVER A STANDARD DIPOLE.
HORIZONTAL COVERAGE IS PROVIDED BY A SINGLE TELECOM TFLHO HORIZONTALLY POLARIZED OMNI FM ANTENNA. POWER GAIN IS 0.93 OR -0.3 DBD.
THE HORIZONTAL ANTENNA IS MOUNTED 9 FEET (1 WAVELENGTH) BELOW THE VERTICAL ANTENNA.
POWER IS SPLIT EQUALLY TO EACH POLARIZATION THROUGH A TELCOM LPD2 FM POWER DIVIDER, RESULTING IN A 0.5X OR 3 DB LOSS TO BOTH THE HORIZONTAL AND VERTICAL BAY OF THE ANTENNA ARRAY.
MAXIMUM ANTENNA GAIN IS IN THE VERTICAL POLARIZATION AT 1.4X OR 1.5 DB.
ANTENNA CABLE IS 225 FEET OF ANDREW LDF4-50A HELIAX WITH A TOTAL CABLE ATTENUATION OF 1.52 DB."

Does anyone have any experience with the Telecom Line of Antenna's such as the above mentioned?

I am considering replacing the antenna's with a single SWR FM1, due to the elevation of the transmitter site, she is licensed to 32 watts ERP each for Horizontal/Vertical polorization.

Antenna Height Above Average Terrain: 51.7 meters
Antenna Height Above Mean Sea Level: 274. meters
Antenna Height Above Ground Level: 18. Metrs

Any input would be appreciated,
Guy
 
The MDR1 is a simple, lightweight folded dipole. Here's a link to the Telecom antenna in question:

http://www.bext.com/pdf/P052-LDR1-MDR1.pdf

Go with the upgrade to the SWR antenna. I have used the SWR low power antennas for translators, they are ruggedly built CP antennas, and will survive much longer in harsh conditions than this lightweight aluminum Telecom antenna. Don't know where you are at, but many of these lightweight "LPFM" antennas won't survive a typical Midwest winter, especially after battling ice and wind. The SWR antenna, by contrast, is stainless steel. Plus the circular polarization will put more field into randomly polarized antennas such as line-cord antennas.

With an ERP of 32 watts, you can quite comfortably drive the SWR through 100' of 1/2 foam with a 100 watt exciter.
 
Tom,
Thanks for the information, including the link. I will utilize the SWR FM 1/1, which seems to be well constructed. It appears to be easily tunable, with the threaded rods on each section. With the 225 ft of ANDREW LDF4-50A HELIAX coax, with 32 watts vertical & 32 watts horizontally (64 watts), a 100 watt transmitter is large enough?
Guy
 
I would agree with Tom; I've seen one of the Bext/Telecom folded dipoles firsthand, and while they might be an economical choice for a temporary translator site, they aren't very rugged.

The SWR CP bay would be a lot more durable, and PSI (which is also in Ebensburg, PA) makes a heavier version of the "Double-V" design with fatter elements which are less affected by minor ice buildup but lack the adjustable end caps. I've used these on several projects and they work well. They can also be ordered with de-icing heaters if you're in an area that gets severe winter weather.

225 feet of Andrew LDF-4-50A 1/2" line has a rated loss of 1.48 dB in the center of the FM band, so for a 100 watt input, you'll have about 71 watts remaining at the antenna input. A single CP bay has a power gain of about 0.46, so this would allow an ERP of 32 watts, but very little headroom.
 
I calculated 101 watts for 32 watts ERP, but that was with an antenna gain of .45. They test each antenna, gain figures for my last two FM 1/1's fell in this range.

Close enough for an LPFM--unless, of course, there is a generous budget for the project. Except in extreme weather, (or on a very high and exposed tower, which this does not sound like) the FM 1/1 should do just fine.
 
The main reason I am asking about the 100 watts is we found a Crown-100T with audio processing and stereo generator. This would be a upgrade from her current gear and at a decent price.
Thanks again,
Guy
 
Guy Betten said:
The main reason I am asking about the 100 watts is we found a Crown-100T with audio processing and stereo generator. This would be a upgrade from her current gear and at a decent price.
Thanks again,
Guy
Click to expand...

A nice service Crown offers is "unit renewal" of used equipment:

http://www.irec1.com/CurrentUserUpgrd/Upgrade4.htm

Under the renewal service, they rebuild the transmitter and give it a new 24 month warranty at a reasonable price. A couple of years ago, I sent in a 500 watt transmitter that I bought cheap at a hamfest for use at one of our translator sites. It was making less than 50% power, and after removing the bottom cover I noticed some of the electrolytic caps in the supply were so old they had split open. It came back working like new, tested on my frequency with all defective parts replaced and upgrades added. They even installed a brand new front panel as well as shiny new top and bottom covers.

I also had them install the new GoRx3 receiver card (the unit was originally equipped with a stereo generator but no receiver) which did develop a problem a few months ago, however they promptly sent me a replacement under warranty. They're good people to deal with.
 
Crown makes nice equipment, but I have had a problem with intermod at one site when I had to bring in a Crown translator temporarily--with a spur appearing on Channel 5. The Crown was set for 5 watts, the other unit was an Armstrong 100 watt, I presume there was some mixing in the Crown's output circuits creating the spur.

Never pursued this issue as the original Tepco equipment was repaired & returned to the site. But if you have a shared site you may need to add a cavity to avoid this problem.
 
TomT said:
The MDR1 is a simple, lightweight folded dipole. Here's a link to the Telecom antenna in question:

http://www.bext.com/pdf/P052-LDR1-MDR1.pdf

Go with the upgrade to the SWR antenna. I have used the SWR low power antennas for translators, they are ruggedly built CP antennas, and will survive much longer in harsh conditions than this lightweight aluminum Telecom antenna. Don't know where you are at, but many of these lightweight "LPFM" antennas won't survive a typical Midwest winter, especially after battling ice and wind. The SWR antenna, by contrast, is stainless steel. Plus the circular polarization will put more field into randomly polarized antennas such as line-cord antennas.

With an ERP of 32 watts, you can quite comfortably drive the SWR through 100' of 1/2 foam with a 100 watt exciter.
Click to expand...

Tom,
I know the SWR FM-1/1 is tunable with the threaded rods, how far across the band have you adjusted them? I am still looking for a manual online.

Guy
 
I have the manual that came with one of my client's FM1/1. The manual does not outline retuning procedures - it only states that it can be re-tuned in the field. The manual is pretty basic and covers the installation and maintenance procedures.
 
Tech support is questionable at best. The best answer I could get is "mail it to us and we will tune it" I will borrow a antenna analyzer and work on it this weekend. I assume the threaded rods need to be lengthened as you tune it for lower frequency's?

tjm_pro said:
I have the manual that came with one of my client's FM1/1. The manual does not outline retuning procedures - it only states that it can be re-tuned in the field. The manual is pretty basic and covers the installation and maintenance procedures.
Click to expand...
 
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