FM Antenna Tuning

FM does it with frequency!
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W9PXZ
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FM Antenna Tuning

Post by W9PXZ » Mon Aug 26, 2019 10:53 am

In the process of prepping the site to install a new GatesAir FAX40 I worked all day Saturday to remove an old patch bay, install a coax switch, install a BDI power monitor and switch controller, and re-plumb the retiring Continental back in. Ran on the aux site for the interim. After getting the Continental 816-5C back on line I noticed that the new BDI metering shows a reflected power that computes out to a VSWR of 1.37:1. Downright awful! No wonder the exhaust stack temps always ran high and it was always a bear to tune after a tube change. I never had a working power meter on this station other than the relative one on the TX. It reads about what it always did for reflected power.

I am thinking an antenna tune is in order. My tower guy is good at what he does, but does not have the test equipment to tune an FM antenna. I own a Rigol spectrum analyzer with the tracking generator and the matching VSWR bridge. Would that be sufficient to tune an 8 bay about 950' up? Would the TG signal be sufficiently strong enough to give valid results? I am also working under the assumption that the other 100 kw C that is mounted right underneath the antenna in question would need to be off during testing as well. The antenna is a mid 80's vintage ERI.

Who knows, it may have been this badly tuned its entire life...
Mike Dinger
Wilmington, NC

DaveSt
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Re: FM Antenna Tuning

Post by DaveSt » Mon Aug 26, 2019 2:48 pm

You would certainly want all transmitters any where near that system to be off before you connect your spec analyser to the system. Test with plenty of attenuators first. Without knowing some details of the antennas, it is hard to comment further. You might have an issue that is not down to tuning of course. You do have the possibility to terminate the feeder and test that with your spec analyser/tracking generator. Likewise you could terminate each splitter output in turn to see if you have a faulty antenna. If it is a big system, a call to ERI would be the best thing to do.

fm-engineer
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Re: FM Antenna Tuning

Post by fm-engineer » Mon Aug 26, 2019 4:43 pm

Echo DaveSt, contact ERI first. They may have some info on it. ERI tunes antennas off center for icing, but that seems a little high. However, looking at reflected power / VSWR is just a quick snap shot, and doesn't tell the whole story. Many older ERI's around here are running at 1.25 VSWR, but if you looked at it on an analyzer, it would carry that for several Mhz, rather than a perfect 1.01 that's on channel but very narrow. Seems the newer ERI antennas are even broader. I have a new SHPX 4 bay that is about 9 MHz wide, and -43 dB on carrier. If it was tuned perfectly on channel it would be about -48 dB.

Also remember the issue may not be with the antenna. It could be something with the line or a bullet /connector. Have your climber disconnect the input to the antenna, install a calibrated load, and then sweep the line. See what the line looks like first before anything else. Most likely, its slug tuned antenna, so you can't just go up and quickly make changes. Very few of these had the fine matching sections installed because the antennas are so broadband.

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kkiddkkidd
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Re: FM Antenna Tuning

Post by kkiddkkidd » Mon Aug 26, 2019 6:05 pm

You could also be seeing some of the other 100kw signal as reflected power. Ask the other station to reduce power to 50% for a few minutes and see if your reflected power drops. If so, your new power meter is reading their power coming back down your feed line.
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W9PXZ
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Re: FM Antenna Tuning

Post by W9PXZ » Mon Aug 26, 2019 6:36 pm

You may be on to something there, Kevin. The other station has a big gray ERI barrel sized filter in its line to pass its frequency and reject the other. This station has one of the filters, but it is not inline. Why it is not, I have no idea and no one is left that could answer that question. Both stations are commonly owned, so maybe late one day I'll do a quick cut on the 2nd FM and see if that has an effect on the reflected power.

If it does, then I will have to see if I can do some re-plumbing to get that filter back inline.

I ordered a reducer today to connect my SA up to the antenna. The selling point was that it will also enable me to connect the market's 1 kW floater up to the main antenna for emergencies. Nearly 4 kW at 950' beats 1100 w at 90' any day.
Mike Dinger
Wilmington, NC

DaveSt
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Re: FM Antenna Tuning

Post by DaveSt » Tue Aug 27, 2019 12:44 am

If the filter is not inline, then it could have been removed because it makes the VSWR problem worse. That does not necessarily mean the filter is not correctly tuned as changing the length of the feeder will affect the VSWR shown at the transmitter. If you are getting enough RF from the other transmitter into this transmitter to affect its VSWR reading, then you will have a serious (reverse) intermodulation problem. I would say that needs to be investigated as a matter of urgency. What are the frequencies of the 2 transmitters?

Kelly
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Re: FM Antenna Tuning

Post by Kelly » Tue Aug 27, 2019 6:36 am

My guess is that filter is a notch, probably to reduce or eliminate an intermod product from being created in your amplifier from another local station 'turning-around'. If there is no data plate on the filter indicating the notch frequency, ERI probably has a record of it being sold to your station.

I second the comment about the higher VSWR being a failing bullet(s), or possibly water in the TX line or antenna. It's pretty unusual for antennas to require re-tuning. I had to re-tune an older RCA/Dielectric years ago, because over time the hoops got dinged and bent from ice shedding off the tower. In that case, we pulled the antenna down, replaced all the bullets, and thoroughly cleaned every connection with Scotchbrite and electronic cleaner. Then using an Agilent network analyzer; I re-tuned each separate bay on the ground (sawhorses) to match the original factory impedance for the frequency. Finally we fully assembled the antenna strapped to sawhorses on the ground and swepped the input to make sure it was 50 ohms on frequency. Then we disassembled and reinstalled back on the tower with all new O-rings. Fortunately, I had zero reflected power and the antenna was deemed good for another thirty years.
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rich wood
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Re: FM Antenna Tuning

Post by rich wood » Mon Sep 09, 2019 11:36 am

If the sample for the reflected is at RF level, Not DC, you could put a 1/4 wave stub on a T connector in line to shunt out the offending signal. FM antennas are very broad in nature and they receive as well as transmit. The tuning section is actually a line matcher that fools the transmitter and does not really tune the antenna, unless you move the gamma straps at each bay. The is possible with some Shively and Dielectric antennas but not ERI.
A spectrum analyzer with a return loss bride is a good measuring tool, but as already said all RF needs to be off or the analyzer will be damaged. The reading you get is the lump reflection at the bottom end and really does not identify the location. This is where a Network Analyzer with Time Domain capability comes in. Short of that, you could open the top of the line and with the bridge measure the line open,,, then shorted ,, subtract the two readings and this shows the insertion loss. By checking the manufacturer's specs, add a bit for connectors,, you get an idea of the line condition.

W9PXZ
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Re: FM Antenna Tuning

Post by W9PXZ » Mon Sep 09, 2019 9:50 pm

Just to update the thread, this Saturday we'll pull the top bay to the ground to make repairs of a lightning hit that ripped a hole in the bay. I'll shut down the other class C that has the antenna located directly underneath this one and take some measurements with my SA/VSWR bridge combo both before and after the repair. The tower crew has an older Anritsu Site Master. It will be interesting to compare the results between the two measurement methods.

I have watched the reflected power while backing the power down on the other station to the minimum its old Continental would be happy with with no change in the reflected power. I'll check the the amount of "undesired" signal while the antenna for this station is being worked on. I have a directional sniffer slug just for this purpose. I am betting that it will be minimal.

The initial high VSWR has calmed down considerably since the original post. Down to about 1.15. Good, but it could be better. The tower crew will be armed with a roll of the 3M 471 tape and prepared to do some successive approximation movement on the slugs. (ERI antenna) We'll see if we can get the center more on frequency while maintaining a nice symmetrical bandwidth.

BTW, the coax for this station was replaced in 2017 with new 4". The old one had suffered a bad burnout and had to be scrapped.
Mike Dinger
Wilmington, NC

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kkiddkkidd
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Re: FM Antenna Tuning

Post by kkiddkkidd » Tue Sep 10, 2019 4:50 pm

Along with the 3m tape, send some different colored sharpies as well...
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kkiddkkidd
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Re: FM Antenna Tuning

Post by kkiddkkidd » Tue Sep 10, 2019 4:57 pm

If you are really into self flagellation... You can do the measurements and calculations yourself.

http://www.scott-inc.com/html/eri.htm
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Kevin C. Kidd CSRE/AMD
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AM Ground Systems Company
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radio_guru
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Re: FM Antenna Tuning

Post by radio_guru » Wed Sep 11, 2019 11:54 am

That was a really informative read. Luckily, I have a HP8751 and HP directional coupler to effect the sweeps needed.

RG

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