tech stuff – Page 34 – Engineering Radio

Water and RF do not mix

Last year, the main antenna and transmission line for WSPK was replaced. I was, therefore, somewhat surprised to hear that there was an issue with the new transmission line. And yet, problems there are. Most likely, some ne’er do well has shot the transmission line with a bullet making a hole, which, when it rains, allows in water. Said water then accumulates in the bend at the bottom of the tower. When enough water is present to fill the gap between the center conductor and the outer conductor, this happens:

Foward and Reflected power meters, WSPK, Mount Beacon, NY

For those of you keeping score at home, that’s 980 watts forward, 375 watts reflected or about 4:1 VSWR. Obviously not a good load, in fact the transmitter shut down. Fortunately, the backup transmitter and antenna system worked flawlessly.

This began happening last month, usually after a heavy rain storm. Thus, I went out to the base of the tower and shook the transmission line and sure enough, water was sloshing around in there. Last time time it happened, a tower crew was summoned to inspect the line. Inspect it they did, but did they find any holes? No, they did not. Perhaps the issue is with the antenna itself, in which case the entire thing will have to be removed from the tower and lowered to the ground. In the mean time, my boss drilled a small “weep hole” at the bottom of the bend where the line comes off the tower.

I uncovered this weep hole and pressurized the line and viola, lots of water came out:

WSPK 1 5/8 inch air dielectric transmission line, Mount Beacon, NY

A bit unconventional, but effective nonetheless. The first video is of the water dripping out:

The second video is of me walking back into the transmitter building to pressurize the line:

Everything is very noisy because it is Monday, when all the generators on site exercise. There are five diesel generators running while I was videoing recording this.

I would estimate about 6 ounces of water came out through the weep hole, most of it landing on the ladder underneath. After the water was drained out, the transmitter came back on at full power and normal VSWR.

A temporary fix to get the station back on the air. The real repair work will begin when the antenna comes down to be inspected.

Update: The tower climbers did find a hole in the transmission line, just below the flange that connects to the antenna. It looks like a pencil sized gash just before the line bends back to the tower. Lightning? Rifle bullet? Damage while installing? We can’t really tell. They installed a patch over the hole which holds about 3 psi line pressure. We then used a vacuum pump to evacuate the line, then recharged with dry nitrogen.

Regarding Pedro’s question below in the comments: Since we found this problem quickly and were able to evacuate the line, there should not be any corrosion, that is our hope. Time will tell

Orban Optimod 8000

A piece of vintage gear from the late 1970’s, the Optimod 8000 was and still is a good sounding box. I have often thought that these processors would make an excellent internet audio processor using the test jacks on the back of the unit. The audio on these jacks is unbalanced and has 75 µS pre emphasis. It would be easy enough to make a de-emphasis network and create balanced audio with a 10K:600 ohm transformer. Some experimentation may be required with the transformer primary impedance value. Orban notes that not less then 1 MΩ impedance should be connected to the test jacks. For the internet station looking to copy the “FM radio” sound, this unit would do the job nicely.

The 75 µS de-emphasis network would look something like this:

75 microsecond de-emphasis network, unbalanced to balaneced audio conversion — 75 microsecond de-emphasis network, unbalanced to balanced audio conversion

In this case, the values for the de-emphasis network are fairly critical, therefore 1% or better tolerances for the resistors and capacitors is required.

Even better, an LPFM or some other radio station on a budget could acquire one of these for relatively little on eBay or somewhere else. With a little TLC, most of these units can be rebuilt and put back into service. I would recommend that some type of limiter be used in front of it, such as a Texar Audio Prism or CRL SEP-800.

Some classical music stations prefer these units. I have noticed that they have a nice, mellow, open sound. Not at all fatiguing and yet still offer a nice easy 10 dB gain reduction. There is also a modification that can slow down the release time on the gain reduction. More gain reduction, AKA compression, can be had with something else in front of the unit.

The best part about these units, there is no rebooting, no processor lock ups, software glitches or any of that non-sense. Additionally, a quick look at the front of the unit shows very few user controls, making it almost impossible to screw up and sound bad. They are well built and so long as the electrolytic capacitors are changed out, fairly bullet proof. Other processors, not so much.

This is an Optimod 8000A that I decided to put through its paces.

Really, how much more do you need? I recorded this on the camera microphone using a replica table radio, seen near the end of the video on the right hand side of the frame.

I used the Technics SL-1200 MKII turntable through an ATI P100 turntable preamp into the Optimod. The Optimod is feeding a BE FX-30 exciter running 15 watts into a dummy load. The Optimod is running about 5-7 dB gain reduction, which is enough in my mind. The BE FX-30 is still just about the best sounding analog exciter every made.

This unit has been re-capped and re-chipped at one point. The re-chipping follows the Orban recommendation; the 4558 and 1556 opamps are replaced by TL071CP and TL072CP respectively, and the uA 709 and 301A opamps are left in the unit. A good thing to remember, the uA709 and 301A opamps can be replaced by TLO71cP opamps in the event of failure. The Texas Instruments TL0 series opamps are very good and readily available.

Overall, this unit is in good condition, however, like many such units, it is missing its brown “Optimod” cover, which goes over the input/output controls.

Manual is available at the Orban ftp site: ftp.orban.com.

WKZE transmitter site

We rebuilt this station’s studio couple of years ago. WKZE is licensed to Salisbury, Connecticut. The transmitter is located near Millerton, NY and the studio is in Red Hook, NY. Today, I had to go to the transmitter site to fix a problem with the station’s processor. While there, I snapped a few pictures:

WKZE, 98.1 MHz, Millerton, NY — WKZE, 98.1 MHz, tower near Millerton, NY

The WKZE single-bay antenna is mounted on the left-hand tower.

WKZE single bay Shively 6810 antenna with Radome

The transmitter is a CCA FM3000DS, made new in April, 1970:

WKZE 98.1 MHz CCA transmitter — WKZE 98.1 MHz CCA FM3000DS transmitter

The CCA designs are dirt simple. Grounded grid, driven with an external solid state amplifier that is a modification.

WKZE CCA transmitter, 42 years on — WKZE CCA FM3000DS transmitter, 42 years on

As you can see, this transmitter was originally manufactured for WHVW-FM, which is now WCZX. The station has a large, mostly cult following throughout the mid Hudson valley. Even though it is a 3,000-watt class A station, its coverage carries far beyond its theoretical 60 dBu contour:

This is because the station is in mono.

The Polyphaser IS-PT50HN-B

I found this on the floor at an old transmitter site:

Polyphaser IS-PT50HN-B DC block surge suppressor

Since it appears to be discarded, I ignored the dire warnings and opened it up to look inside:

This is is a DC blocked lightning surge suppressor designed for 890-980 MHz, 750 watts maximum. The two parallel wires represent a capacitor, coupling the radio to the antenna, the inductor acts as an RF block to the gas discharge tubes. The design is such that the inductor acts to block the normal in-use radio frequencies but will allow the 10-30 KHz lightning pulse to pass to the gas discharge tubes and thence to ground. The inductor and gas discharge tubes are on the antenna side of the unit. I measured these units with a DVM and they all appear to be good.

My only comment on this unit is that there is no effort to maintain the transmission line impedance. At the upper end of the UHF spectrum, this can lead to return loss and wasted power. For a receive application, it may not be so bad, but for a transmitter, I would rather use something else.

For lower VHF frequencies, something like this can be DIY fabricated with minimal expense and effort. The case must be bonded to the station ground.