Tuesday, September 04, 2007

In the Eye of Felix and Some Tentative Tones

Just a little note to let you all know that we're safe here. So far Felix has only dumped a bit of rain, threatened to blow the tin roof off of the transmitter shed, and made the usual light-jacket evening fairly cold, well really only probably in the mid 50's. It hasn't been so nice for the folks on the east coast though. First reports are just coming in to HRN, the national radio system. A lot of people live in just basic wood houses on stilts in the swampy rain forest there -- not a nice place to be in a category 5 hurricane. Tegucigalpa is bracing for a lot of rain and the mayor has evacuated the central market and steep regions of cobbled-together housing that washed away before in Hurricane Mitch. Tegus is in a narrow valley between mountains with kind of unstable slopes. The eye of the hurricane is expected to pass right over us about mid day tomorrow, but by then they're thinking it may be downgraded to a tropical storm.

Today, as Felix's winds were blowing in, I ran the first actual transmission on HRJZ, The Voice Lenca, 1580 kHz AM. Well, actually it was just a test tone, followed by a pretty distorted re-broadcast of the FM signal. The problem was that there was interference in the FM receiver from the AM transmitter being so close -- sort of a Catch 22 that can be solved by a better antenna for the FM receiver or by installing the microwave link that is the eventual design anyway. And, it's still running at just 250 watts out of the licensed 1000 watts. Between power outages and rain that have interfered with the main goal of getting the transmitters up and running at full power, I've been able to catch up on the lower priority jobs like designing the antennas for the microwave link, which need to be done before leaving.

I've got some problems with the tuning on these transmitters and the ATU as well. I'll put a technical discussion of that in the comments for this posting, so as not to clutter up the blog with technotalk.

That's it for now. I hope you all doing OK in the heavy heat that shouldn't be happening in Sept. in Illinois.

La Lucha Sigue! Bill


Bill Taylor said...

I thought my tuning blues were over, but I've gotten stuck on some things.

I'm working with two fairly identical Harris MW-1 transmitters, feeding into a Harris ATU, then into a new quarter wave tower. Both of these transmitters were on 1400 kc., and I'm coverting them to 1589 kc.

Running into their internal dummy loads, I'm just able to get one of them up tp 1 kw by careful tuning, but getting that to load into the antenna is another matter.

Following Harris's frequency-change instructions, I changed the capacitors just after the bank of PA tuning coils. They had been 0.03 mfd, and I changed them first to 0.018 mfd (a bit lower than Harris recommended) to get a null PA volts reading within the range of the PA tuning slugs. However, with subsequent tuning the PA tuning is still at the end of it's range, with the slugs all the way retracted. Do I just keep reducing the capacitance? I've tried substituting caps down to 0.015, but it just gets unstable.

Another problem is that the four 91 ohm, 2 watt resistors on the Directional Coupling board are getting really hot. They haven't opened up yet, but when I checked them they were all at about 122 ohms, so I put some other big resistors also in parallel to get the array down to about 23 ohms, but they still get hot when I'm going through the tuning. Since these form the main part of a voltage divider across the output of the current sensing transformer on the final output, I'm wondering if I'm frying these out by improper tuning. (This is happening on both xmtrs,) The ammeter is showing only about 12 amps (at 250 watts out), well below the 22.5 amp current that it pulls at 1000 watts.

Another thing that's got me bugged is the nulling out of the rel/swr circuit. It's requiring way bigger caps. across the nulling variable capacitors than described in the instructions. I've got 174 pf across the left hand one [C23] now, and it only nulls at just slightly above being screwed all the way down.

In addition to all of this, the WGIL xmtr is gyrating wildly when I tune it. Looks like parasitics or something really loose. Both xmtrs are taking way too much turning of the PA voltage pot to get up to the initial 250 watts, leaving not enough turning left to get the xmtr up above 500 watts. Am I doing something really wrong here?

Have I got the ATU completely mistuned? [I've been using the reflected and forward power meters on both xmtrs to lower the swr on that] About the lowest I've been able to get is 25 watts reflected at 250 watts forward power. I've been using the original coil in the ATU, substituting different cap's in L-networks and T-networks, using the two ends of the coil as the input and output coils of the T. I think the coil might be too big -- I calculate it to be about 95 microhenrys for the whole length. It also doesn't help that I only have 0.04 mfd, 0.01mfd, .002 mfd, and 0.0001 mfd. caps to choose from, but I can get some different values from combinations of these.
When we got it, the only functional parts were that big coil and a 0.002 mfd cap. with one pole connected to ground and the other to two flexible clip leads which ere tapped into the coil, making a T-network, with the input and output coils being the two opposite ends of the big coil, thus magnetically linked. I'm not sure how that linkage would affect the resonance of the T-network or the input and output impedances, but we're trying to match to a vertical with a base impedance of about 33 ohms (1/4 wave, 90 radials)

I've been at this tuning business for over a week, and I really don't know how to clear this up, but I'm really ready to go back home.

Thanks so much for your help!!

Peace, Bill

Dominic said...

My wife (Kerstin) and I visited La Esperanza in early August. It was great to meet the people at COPINH and La Voz Lenca. It great that they can now use the AM transmitter to communicate with communities in outlying areas.