PRESSURIZING FM ANTENNA THROUGH FOAM HELIAX

[ABQRADIO]

I am constructing a new FM station which will use an 8-element pressurized PSI antenna, and have an inexpensive local source for new 1 1/4" LDF6-50 Foam Heliax. Has anyone used foam coaxial cable to feed a pressurized antenna, using the tubular center conductor to carry the pressure to the antenna? I am wondering if one could use the pressurized connectors with that cable to get the job done. Your comments and help will be greatly appreciated!

 

[frankberry]

As far as I know, you can't pass pressure through foam Heliax. You need air dielectric Heliax. You could possibly run tubing all the way up the tower but it would be expensive and the tubing might not last too long if exposed to direct sunlight. Your best bet is to use the proper Heliax.

 

[rfburns]

I've never heard of it being done, but I suppose you could modify the center conductor to pressurize the antenna. I've always used air dielectric line though.

RFB

 

[littlejohn]

Is there anyone else on the facility from whom you might buy pressure at the top of the tower?

 

[WNTIRadio]

I'm almost finished with a new FM construction myself. The transmission line isn't really the most expensive part of the job at all. Why would you cheap out at this point and risk blowing up the transmitter and/or burning out the antenna just to save a few bucks on the transmission line?

How much did you pay for the PSI antenna? The transmitter? I'm sure either weren't cheap. Doing what you want to do is like putting almost bald used tires on a brand new car. Spend the money, borrow the money, beg for it to buy air line and do it right the first time. The few hundred dollars you're going to save now will come back to bite you in the ass big time in a year or two when the tower riggers have to go up and pull down all of your foam line (and maybe an antenna bay or more) and replace it with air line because the job wasn't done right in the first place.

This is a NEW FM. You have a chance to do everything right the first time and avoid plenty of headaches down the road. If the owner isn't you and he's the one cheaping out, explain to him, very carefully, how a few more dollars now can save thousands later on. Not to mention lost air time.

 

[ABQRADIO]

Wow, WNTI, I feel like I just endured a lecture on fast food from Michelle Obama I really did not intend to ruin your day.

Blowing up the transmitter and burning out the antenna? Good grief, as long as I can safely keep dry pressure on the antenna what does it matter how I do it? And I remember sometime in the past 40 years in radio I talked to someone who was successfully doing what I am proposing.

Indeed, I am the owner, with 100% of the money coming from my wallet. And this will be about the 25th station I have built for either myself or others, AM, FM, LP's to flamethrowers.

Now that I am done stick-poking, I probably will go with airline.

Thanks to Littlejohn, RFburns and Frankberry for their responses. Agreed on the tubing comment from frankberry.[size=12pt]

 

[DTV-Chief]

Wow, WNTI, I feel like I just endured a lecture on fast food from Michelle Obama I really did not intend to ruin your day.

Blowing up the transmitter and burning out the antenna? Good grief, as long as I can safely keep dry pressure on the antenna what does it matter how I do it? And I remember sometime in the past 40 years in radio I talked to someone who was successfully doing what I am proposing.

Indeed, I am the owner, with 100% of the money coming from my wallet. And this will be about the 25th station I have built for either myself or others, AM, FM, LP's to flamethrowers.

Now that I am done stick-poking, I probably will go with airline.

Thanks to Littlejohn, RFburns and Frankberry for their responses. Agreed on the tubing comment from frankberry.[size=8pt]

I'll agree with WNTI that the tried and true industry-standard of using air dielectric feedline to pressurize the transmitting antenna is the way to go. Why would you want to reinvent the wheel after years of proven reliability at countless other FM or TV stations?

 

[WNTIRadio]

I wasn't coming down on you in particular, it's just that you have a golden opportunity to do it once and not have to think or worry about it. I've spent too much time undoing the mistakes of others... yes, it does keep me in business, but it's nice to see a facility done correctly where the owner can worry about the programming and marketing of his station and not have to keep running up the credit cards to fix technical mistakes.

Didn't mean to come down on you like a ton of bricks.

 

[WNTIRadio]

Also, to clarify the "blowing up a TX or antenna"... all it takes is a little bit of water intrusion to cause a short or intermittent short in the right places. Have seen and replaced burned up antennas, transmission line, switches and amp modules in transmitters. Most of those were due to either lightning or water getting in where it's not supposed to be.

 

[kenglish]

Just be sure the air line is made of something that is resistant to UV light, and put an interlock on there that will kill the transmitter if there's a leak.

 

[bradgoehl]

I have done it and it doesn't work well. It eats nitrogen about a bottle a month, and the reflected was never quite right. The attempt was made to save money and use spare gear to get a station back on after hurricane Ike. Thankfully we are about to act on our CP to upgrade and move the station. Especially if your buying new, Just get the correct air line.
Just my .02

 

[OKCRadioGuy]

Sometimes a "deal" really isn't in the long run. In theory you could run air directly to the antenna, but I think you're just asking for trouble. I've installed a hairdryer station in a small market that used a unpressurized antenna and foam feedline. It has worked well for them. If you plan on going with unpressurized line, you might consider seeing what type of antenna uses no air in it also. At real low power levels, it's an option. If you're transmitter spits out more than about 1k, I'd sincerly think about just going air all the way and get a dehydrator. If you cut this corner now, you'll be pretty unhappy with the results in the future IMHO.

 

[DTV-Chief]

Sometimes a "deal" really isn't in the long run. In theory you could run air directly to the antenna, but I think you're just asking for trouble. I've installed a hairdryer station in a small market that used a unpressurized antenna and foam feedline. It has worked well for them. If you plan on going with unpressurized line, you might consider seeing what type of antenna uses no air in it also. At real low power levels, it's an option. If you're transmitter spits out more than about 1k, I'd sincerly think about just going air all the way and get a dehydrator. If you cut this corner now, you'll be pretty unhappy with the results in the future IMHO.

Our two transmitting antennas have 1/2-inch copper return (or bleed) lines that run from fittings at the base of each antenna down and into indoor gauges and shutoff valves and fittings. This allows monitoring pressure at the antenna plus supplemental charging of the antenna or remedial purging of the line. Something like this could work backwards to pressurize an antenna having foam feedline.

 

[frankberry]

Installing air dielectric Heliax would cost far less than running copper tubing all the way up the tower. Copper theft might also be a problem.

 

[BobOnTheJob]

I have first hand experience. In case #1, the LDF6-50 existed at a station I took over...that was several years ago. Still using the original tank. In case #2, a new class A was being built on a tower that a 900mhz paging company had previously abandoned...they left a run of LDF6-50 that was of the correct length. Based on the experience in Case #1, the new 4 bay half wave was installed in December 2009 and the LDF6-50 was attached. ERI can get the connectors, but at the time, they were a special order item and took a couple of weeks to arrive. A full size tank of nitrogen was on hand to purge the system. After that, a small nitrogen tank charged the system...it hasn't been touched since and the small tank is still showing full. It takes almost no nitrogen to fill the line as all you need to fill is the center conductor and antenna. One would have to assume that if an errant target practice bullet knicked line, it wouldn't leak either. The SWR is perfect. My experience in both cases has been nothing but great.

 

[ABQRADIO]

Bob, thank you for the benefit of your on-the-job experience. Sounds like you successfully did exactly what I am proposing. And your comment about bullet holes is great - I had not thought of that, but that could be a Benefit of using the foam line as I have had to deal with some very costly bullet hole damage in air line. I will start my research as to suitable connectors and report back. Thanks again!

 

[BobOnTheJob]

Bob, thank you for the benefit of your on-the-job experience. Sounds like you successfully did exactly what I am proposing. And your comment about bullet holes is great - I had not thought of that, but that could be a Benefit of using the foam line as I have had to deal with some very costly bullet hole damage in air line. I will start my research as to suitable connectors and report back. Thanks again!

The connector is an L46R. It's a gas pass connector that will be used at the top and bottom. A separate Gas Block is used at the bottom. The L46R has a gas input port on it. I got mine from Denise at ERI...she should be able to help you. It may be available elsewhere, but since a new ERI antenna was being ordered, my client chose to order the connectors as part of the package. Good luck!

 

[BigRed]

I know that this is an ancient post, project long finished, but I thought I would add my two cents . . .

1) So far as I remember you CAN pressurize an antenna through foam-dielectric line. Almost all foam lines, 7/8" and up, have tubular center conductors and connectors available from the manufacturer that will allow you to take advantage of that option. I've done it in the past using LDF5 to pressurize the feed horns on 2-gHz microwave antennas. Ask the line manufacturer about using that option.

2) If you do use air-dielectric cable then it's a good idea to run a separate line to the antenna. Most commonly used for that purpose is 1/4" or 3/8" poly that is UV resistant (always make sure that the the line is UV resistant) but if you have a few extra bucks in the budget (I know, what's a few extra bucks) run stainless-steel tubing. A rigger can stand on the SS tubing and it probably won't budge; even copper will crush. Separate pressurization helps insure that if one becomes contaminated or develops a leak the other will remain protected.

3) I prefer nitrogen over dry air. The objective is to prevent moist air and water from getting into the line by keeping positive pressure on the line and to also prevent corrosion. Using dry air over nitrogen gives oxidation about an 18% head start. And I have yet to see a dehydrator that didn't fail at some point an blow wet air into a line. If you want to really do it right get a dewer of liquid nitrogen to feed the line for the first rew weeks (equivalent to about 15 nitrogen tanks). Place pop-off valves (that "pop" at above the normal pressure but below the max system pressure) at the top of the line and antenna (put ball valves between the pop-off valves and the line and antenna). After installation run the nitrogen for a couple of weeks at above the pop-off pressure to dessicate the line. After the purge is complete you can have the next climber on the tower close the ball valves to prevent any leakage (but sometimes a little leakage can insure the line stays "fresh").

 

[rfry]

If you can find it, Andrew LDF6-50 (1-1/4" OD foam dielectric Heliax[SUP]®[/SUP]) could be supplied with either L46S 7/8" or L46R 1-5/8" EIA flanged connectors that have a pressure path using the hollow inner conductor of the cable. I think those connectors would need to be modified in order to pressurize an antenna through its input flange, though.

Maybe CommScope (present owner/manufacturer of the old Andrew Heliax lineup) could help.