Gas or no Gas

[Carmine5]

I'm considering building out an FM CP that has come up for sale. In preparing a list of equipment, engineering and specs for the build I've been getting conflicting advice on the the type of coax to use for the antenna.

I was thinking of using Andrew Helax AVA5-50 7/8 virtual air foam dielectric cable with their Positive Stop connectors. They say this combination will effectively seal out moisture. But I'm hearing from an engineer that I should, instead, get Andrew Helax standard air dielectric cable and fill it with CO2.

So which is better for keeping out moisture, gas or no gas?

 

[DRoberts]

There are pros and cons to both types of feedline. The problem with the foam is that if it ever leaks or develops a path for moisture to enter, the foam becomes saturated and will short. Foam will eventually leak. Period.

The Heliax gas pressurized line is probably my choice as long as you have an experienced tower crew install the terminators or connectors. Initially connect a tank of nitrogen to the line to purge it for at least 30 days prior to connecting a dryer/pressurizer.
The nitrogen is not expensive and will reveal any leaks over a period of a month or so after it is connected to the feed line. Insure that you have a pressure gauge that is easily readable and have it read every day or two during the first 30 days. If there is any significant pressure loss, then there is a leak somewhere in the system. As soon as the pressure in the line is stabilized fora week to 10 days then have the system connected to the dryer. Insure that you keep the dessicant changed in the dryer at the recommended intervals and you should have no problem.

(Make sure you use nitrogen gas ONLY. I had a tower installer pressurize the line with Oxygen, yes oxygen. Oxygen does not play nice with high RF and even a small arc. The tower installer's insurance company had to replace 1700' of pressurized feed line due to his helper picking up a tank of O2 instead of Nitrogen.)

 

[chriscollins]

All our sites are Heliax & Nitrogen. Not really my area of expertise, but we never have line problems with that arrangement.

 

[rew]

Another thing to consider is, does the antenna need to be pressurized? If it does be sure to use pressurized line and a gas pass type connector at the top to pass the pressure on to the antenna.

 

[Dale H. Cook]

... and fill it with CO2.

You don't use CO2, you use Nitrogen or dessicated air (which is 78% Nitrogen). You need a pressurizing gas that will scavenge water vapor.

So which is better for keeping out moisture, gas or no gas?

I prefer gas with a pressure monitor connected to the remote control. If the line or antenna sustain any damage, it might cause a pressure leak and the presure alarm will alert you so you can get a tower crew there pronto. You will, of course, have the reflected power monitor connected to the remote control and almost any other damage to the transmission system will cause higher reflected power and alert you.

Another point - if you are using an antenna that is designed to be pressurized you are better off with gas - otherwise you will need to use dessicant capsules to keep the interior of the antenns dry, and changing those out involves a tower crew.

 

[WNTIRadio]

What's the power level of the station and how much transmission line are you running?

Anything with moderate power and a moderate run of transmission line I use pressurized line. Some places I have nitrogen tanks, others I have dehydrators. Both have their advantages and drawbacks. But positive pressure with dry air inside your transmission line and antenna has no drawbacks.

The only place I use foam is for STL transmit/receive and translator receive applications and for short run/low power translator antennas where the antenna isn't pressurized.

 

[Carmine5]

What's the power level of the station and how much transmission line are you running?

Anything with moderate power and a moderate run of transmission line I use pressurized line. Some places I have nitrogen tanks, others I have dehydrators. Both have their advantages and drawbacks. But positive pressure with dry air inside your transmission line and antenna has no drawbacks.

The only place I use foam is for STL transmit/receive and translator receive applications and for short run/low power translator antennas where the antenna isn't pressurized.

It's a Class A with an ERP of 5.4 kW. I would be running 250' of transmission line. But I can already see that I will need to budget for pressurization equipment. Thanks everyone for the information.

 

[Kmagrill]

The ERP is not as important as the TPO. Do you know how many antenna bays (and spacing) you will be hanging or what the gain of the antenna is?

If the TPO is under 3kW, you can safely use 1 & 5/8" coax, either foam or gas, provided that the antenna does not need pressurization. If the antenna requires gas, just spring for the pressurized coax because the alternative is to run a pressure line externally to the antenna, which is not the best idea.

Foam is often less efficient than pressurized, so it's commonly preferred for higher power installations due to electrical cost savings. Foam also has lower power handling capabilities, so if you're right on the edge, you may have to jump up a size. On the other hand, I think foam has lower maintenance costs than gas. I've had some sections of foam line in place for decades and they're still nearly as efficient as they were when they were installed in the 1990s. I've seen some installations where the foam has been in place since the '60's and is still fine. I have never had a foam line leak. It is possible, though rare in my experience. All pressurized lines require monitoring and periodic maintenance. At the very least, seals need to be replaced every 10 years, plus the dessicant has to be baked when damp or you have to rent a nitrogen tank and keep an eye on it. My rule of thumb is 3kW TPO or less = foam. More than 3kW is usually worth the hassle of going pressurized.


Remember that feedline is rated at near zero VSWR. While something like 7/8" line may be rated for your power level at FM frequencies, it may arc if there is any VSWR at all. Typically, for powers between 1kW and 10kW, 1-5/8" is preferred in FM service.

 

[Carmine5]

The ERP is not as important as the TPO. Do you know how many antenna bays (and spacing) you will be hanging or what the gain of the antenna is?

If the TPO is under 3kW, you can safely use 1 & 5/8" coax, either foam or gas, provided that the antenna does not need pressurization. If the antenna requires gas, just spring for the pressurized coax because the alternative is to run a pressure line externally to the antenna, which is not the best idea.

Foam is often less efficient than pressurized, so it's commonly preferred for higher power installations due to electrical cost savings. Foam also has lower power handling capabilities, so if you're right on the edge, you may have to jump up a size. On the other hand, I think foam has lower maintenance costs than gas. I've had some sections of foam line in place for decades and they're still nearly as efficient as they were when they were installed in the 1990s. I've seen some installations where the foam has been in place since the '60's and is still fine. I have never had a foam line leak. It is possible, though rare in my experience. All pressurized lines require monitoring and periodic maintenance. At the very least, seals need to be replaced every 10 years, plus the dessicant has to be baked when damp or you have to rent a nitrogen tank and keep an eye on it. My rule of thumb is 3kW TPO or less = foam. More than 3kW is usually worth the hassle of going pressurized.


Remember that feedline is rated at near zero VSWR. While something like 7/8" line may be rated for your power level at FM frequencies, it may arc if there is any VSWR at all. Typically, for powers between 1kW and 10kW, 1-5/8" is preferred in FM service.

Thanks, Kmagrill. I will be using a TX with a 3 kW output. The antenna system does not need to be pressurized. I spoke with ERI today and they recommended Andrew 7/8 Heliax standard air dielectric cable with a regenerative dehydrator. But I will call them back and ask about 1 5/8" cable and the use of foam coax at low TPO. Thanks again.

 

[WNTIRadio]

I've had the inner conductor on foam sections fill up with condensation from a break in the outer jacket and then pool at the bottom of the tower or other horizontal section of line. Not fun.

You'll have to weigh the options with line size vs. TPO. Larger line means less TPO because of less loss going up to the antenna.

Any class A's I've put in I typically use 1 5/8" air. One site has 3" air, but that was kindly left from the previous MediaFLO installation.

 

[TomT]

Have a similar system to the one you describe using 7/8th air cable--2450 TPO to 200' of 7/8th to a 3 bay for 3200 watts ERP. Let's run through a typical installations for your 3500 watt ERP station with 250' cable run:

Using Cablewave 7/8th air cable, 250 feet has 81% efficiency. Using a 3 bay ERI antenna, then the transmitter power out would be 2760 watts. The Cablewave cable has a maximum power rating of almost 7 kw. So this gives you lots of margin on the transmitter and cable.

The cost or pressurization? Well, you will need gas-stop connectors at each end of the cable, the bottom one should have a port to feed the nitrogen in. Cost difference between these and regular 7/8th EIA flange cables very minor. You will need a regulator, which costs around $250. Your local welding supply shop can supply the nitrogen tank. Been a while, but last time I rented the tank it was around $50 for the gas and a monthly fee (around $10) for the tank rental.
If everything is tight, the tank should last for a couple of years.

1 5/8th" cable would give you extra margins, but at extra costs and extra weight on the tower.

I normally deal with RF Specialties(Ebensburg, PA), they can supply the cable with the connectors attached, as well as the regulator & hose.

 

[Kmagrill]

If the line was properly installed and sealed, condensation shouldn't occur because humid air shouldn't have a way to ingress. But, of course, any line can leak if it's not sealed properly, even foam and a line that's sealed pretty well, can still leak over many years. As has been said, there are tradeoffs in either type of line.

At 3kW, inch and 5/8" air or foam should work well and there's a lot of margin leftover. You should have no problems for many years with either, if they are properly installed. I've not looked at the specs for 7/8" line recently, but I'd certainly sleep more soundly with any 1&5/8" line instead of putting 3kW up a 7/8" line, even if it is pressurized and rated for that much. 7/8" might work very well, but I'd just be a bit nervous every time it stormed or iced up a bit.

No matter which way you go, get a very good crew to install the antenna and feedline and make sure that they seal the line carefully, especially at the top.

Also, unless your manufacturer says otherwise, do not let them hoist the antenna up already assembled, if it's more than 2 bays. Generally, that practice is bad for the antenna and dangerous for the crew. Entire towers have come down as a result of doing that and countless antennas have been damaged or destroyed.

 

[Carmine5]

Well, here's the other option: a station owner at the tower site has offered to diplex his signal with mine. Unfortunately, combiners usually have to be custom built and I'm getting turnaround times from vendors of 60 days. The CP is set to expire on tax day (April 15).

Plus the cost of combiners can be prohibitively expensive. So, ideally I'd like to find a combiner that isn't too costly and will take 3 kW x 2 feeds and be available in a two-three weeks. If you know any vendors that would fit that criteria let me know. The channels are 105.3 and 103.5.

 

[Kmagrill]

You don't have a lot of time to spare, so hanging your own antenna seems to be your most likely option.

If you're really bent on using a combiner, but the time is going to run out on you, then, if the other station is willing, you can hook up their feedline to your transmitter and broadcast for a day while you're still waiting for the combiner.

When you fill out the license app, you are certifying that the station is ready to start broadcasting, which you have demonstrated when the station did the test broadcast. This can be done at any time between now and the expiration of the permit. You are not required to actually commence regular broadcasts until the license is issued, which is usually several more weeks.

NOTE: This only works if their antenna is at the same height as your authorization and if there are no directional conditions to your license or theirs. If anything is different, you have to make a CP mod, first.

If you choose to combine, you will, at the least, end up filling out a new 302 as the TPO will need to change to account for the combiner losses.

 

[BobOnTheJob]

Well, here's the other option: a station owner at the tower site has offered to diplex his signal with mine. Unfortunately, combiners usually have to be custom built and I'm getting turnaround times from vendors of 60 days. The CP is set to expire on tax day (April 15).

Plus the cost of combiners can be prohibitively expensive. So, ideally I'd like to find a combiner that isn't too costly and will take 3 kW x 2 feeds and be available in a two-three weeks. If you know any vendors that would fit that criteria let me know. The channels are 105.3 and 103.5.

Note that not many antennas will accept two frequencies that far apart at the same time. If you're even considering the diplex option, the first thing to do is call the antenna manufacturer and ask if using that antenna on those 2 frequencies is an option. That may make your decision for you. For example, I once ran a 92.7 station into a 91.5 antenna on the same tower in an emergency. Even at that close frequency, the reflected power showed 11% (0-1% is optimal--newer transmitters will start to balk at 10-11% reflected power). It kept them on the air for a couple of days but it would not have been acceptable as a permanent install.

 

[rfry]

... So, ideally I'd like to find a combiner that isn't too costly and will take 3 kW x 2 feeds and be available in a two-three weeks. If you know any vendors that would fit that criteria let me know. The channels are 105.3 and 103.5.

To pursue the comments of BobOnTheJob, which he posted a few minutes before I finished the text below...

Another consideration here is the SWR bandwidth of the antenna (as installed) along with its safe input power rating for this service, and its specified gain at the added frequency. Those numbers need to be supplied by the antenna manufacturer.

The antenna and even the transmission line to it may need to be modified for simultaneous use on those frequencies. The time and cost needed for this may be significant factors for this approach.

 

[Carmine5]

You don't have a lot of time to spare, so hanging your own antenna seems to be your most likely option.

If you're really bent on using a combiner, but the time is going to run out on you, then, if the other station is willing, you can hook up their feedline to your transmitter and broadcast for a day while you're still waiting for the combiner.

When you fill out the license app, you are certifying that the station is ready to start broadcasting, which you have demonstrated when the station did the test broadcast. This can be done at any time between now and the expiration of the permit. You are not required to actually commence regular broadcasts until the license is issued, which is usually several more weeks.

NOTE: This only works if their antenna is at the same height as your authorization and if there are no directional conditions to your license or theirs. If anything is different, you have to make a CP mod, first.

If you choose to combine, you will, at the least, end up filling out a new 302 as the TPO will need to change to account for the combiner losses.

I know the antennas (2-bay) are Jampro JCPB which can be used in a multi-frequency configuration. As for the CP mod (which I will need to do), the FCC said that they can rush through an approval in two weeks if so indicated.

The idea is just to get the station on the air and a license applied for before the CP expires. I don't even plan on keeping it at its current tower site location. Eventually I'd like to move it, if possible.

 

[DRoberts]

If you're going to be up that close to another station (the combiner scenario) be sure to have your engineer run the calculation for potential intermod (remember that an antenna receives while it is transmitting). I built a station transmitter on a mountain in AZ that was the local transmitting site for the area. AFter the station was on the air several weeks, we got a call from the FAA in Las Vegas. It seems that our signal was intermixing with another station on the mountain and we were playing Bob Seeger right into the aircraft radio of a Delta jet.
After immediately going to low power and running the numbers, we were generating a signal right smack in the middle of the aviation band. The resolution was a tuned cavity filter that was pretty pricey but did the job. Only problem was that we had to file for a modification and tweak the transmitter to compensate for the lossy tendencies of the filter.
One of the jocks found our about it and made a big deal about it on air.....his close out on his show for the next few months was...
"station" to Delta 385, climb and maintain Flight Level 370..... None of the listeners really knew the real deal....but we sure did!

 

[Kmagrill]

I know the antennas (2-bay) are Jampro JCPB which can be used in a multi-frequency configuration. As for the CP mod (which I will need to do), the FCC said that they can rush through an approval in two weeks if so indicated.

The idea is just to get the station on the air and a license applied for before the CP expires. I don't even plan on keeping it at its current tower site location. Eventually I'd like to move it, if possible.

Uh Oh. The math doesn't add up. A 2 bay fully spaced antenna has a gain of very close to one, not two. If your ERP is 5.4kW, you need 5.4kW at the antenna input. If the feedline's 90% efficient at the length you're using (only a rough guestimate), then you need a TPO of 6.0 kW to make licensed power. For the transmitter to have 3kW TPO and an ERP of 5.4kW, you need no less than 4 fully spaced bays.

You can achieve licensed power using a pair of horizontal only bays, but I wouldn't really recommend anyone do that if there's any reasonable alternative. Horizontal-only polarization leaves a lot to be desired in coverage, especially for car radios.

Something else to consider is the combined power and its effect on the antenna and feedline. While inch and 5/8 coax is fine for one station running even up to 6kW, putting 12kW into the system might be getting close to the edge. I'd check with Jampro and the feedline maker about the power limits. If you're even close to the edge, you should not attempt it. If you combine the two stations and the feedline or antenna does fail, even if it's not your fault, they're going to still believe it is your fault. Get a hold-harmless as part of your agreement.

 

[Carmine5]

Well, we haven't firmly decided on a strategy yet. We may still go with a 2-bay antenna system. But we'll have to add an extension on the tower for that. I'd like to get 3 dBd gain, if possible. But time is running out so we will need to make a decision before the week is out.