Antennas

[-mjk-]

Fixed. Sorry about that.

 

[-mjk-]

That's a great story, Jerry. Sounds like a lot of fun! I will tell you about the 2 most devious foxes I've heard of. One by someone I personally know and the other told to me by one of the hunters.

In the Southwest there was a foxhunt that started on a small mound. The hunters were gathered at this mound and everyone had a clear copy on the fox. The problem was, as soon as anyone left the mound, the signal disappeared completely.

In the above case, the fox was hidden in the mountains, many kilometers away. The owner was running the legal power limit of 1,500 watts into a directional antenna (yagi, mounted vertically) and had used a telescope to align the antenna so the major lobe would intersect the top of the mound. As soon as one walked down the mound, they would move out of the aperture of the antenna. The fox wasn't found until weeks later, when someone picked up the signal near the mountains.

The goal of whatever camouflage technique used is to fool the hunters, of course. Since the fox is mobile, it is possible to get a fix on the direction. But, terrain blocking is a very effective way to home in on the fox. If you have the signal and you are watching the S-meter, when you pass a building or other obstruction you may observe a jump in the signal. That can give you a useful point for triangulation. But that technique assumes that the signal is consistent.

Automated foxes usually come on the air once every few minute or so and only give the call sign in CW, then go silent. Others may broadcast for longer periods. But there typically is silence. So it takes patience because often moving too much between broadcasts can cause problems with locating the fox. It's always easier to find a continuous signal of course.

John, WN9T mounted a 2-meter transmitter fox in a tree on the WILD property. He mounted a directional antenna on some kind of a swivel and then attached a wind vane to the antenna mount so it rotated with the wind. At any given stationary point, monitoring the signal showed it moving up and down as the antenna swung around. That made it impossible to get a fix on it by terrain blocking. And, if the antenna turned itself around and showed you it's back, you didn't get a signal at all. You could literally be a block away and not hear it and move somewhere else further away before you picked up the signal again. Simple but brilliant. That one took weeks to find also.

 

[skier]

Thanks for fixing the shortwave videos! It sounded as if the interview was taking place in the UK because it was UK-centric, was it? And am I correct that you were receiving it in Taiwan? I'm asking because you were receiving it on the 40m band and the distance would be over 6,000 miles - that's a 3-hopper (my word) and I found it remarkably clear and strong for a 3-hopper.

 

[-mjk-]

Jerry, I was in Chiang Rai, Thailand when I recorded those clips. I was using the Sangean ANT 60 longwire antenna that came with the radio:

And yes, that was the BBC but who knows where the relay station was/is? The word TAIWAN that is visible on the radio is the name of the memory page. Those names make grouping memories a lot more organized. I just happened to have been on that memory page but was manually tuning the bands when I ran across that show.

 

[skier]

Well Gee, MJ, Chiang Rai is only 5,500 miles away. HA!

Actually, while I was more focused on the frequency displayed and not the word TAIWAN on your radio, the thing I hadn't considered was that the signal might have come through a repeater. Too bad, I was just so impressed with the idea of a high-quality signal at 3 hops - that's near the F2 layer maximum - it had me excited. Now, it's just another program received from a nearby relay... SHEESH!

 

[-mjk-]

I have an SWL app on my Android phone. It does show the path for many of the stations. I'm sure that info is publicly available. I recorded those videos just near midnight local Thailand time.

 

[Peter Batah]

View attachment 2179

I thought members following this thread might enjoy seeing this ATU so large that you walk into it. I don't know for which station this unit serves, but it broadcasts a 250KW signal. The inductors and tank capacitors are some of the largest I've ever seen. The copper ceilings indicate that this room is a true Faraday cage.

Have no idea what this is. But, I am impressed for some reason!

 

[skier]

@Peter Batah, you're right to be impressed! (Warning, long, somewhat complete explanation. Read at your own risk!)

When a signal is sent out from a transmitter to an antenna, the power will only be completely transferred and radiated by that antenna if the impedance of the transmission line matches the impedance of the antenna. If they don't match, you get standing waves on the transmission line just as you do with sound in a reflective room and the signal's energy bounces back and force being wasted. In low frequency broadcasting, such as the AM band, the antenna is not able to be placed at the top of the tower because the antenna length is a function of frequency. As frequencies get higher, antennas get smaller. Cell phones can have tiny, internal antennas because the phones operate anywhere from 700MHz to 2.5GHz, depending on the carrier.band (Verizon, T-Mobile, Sprint, etc.) The wavelength of the signal is the inverse of the frequency - frequency goes up, antenna gets short - frequency goes down, antenna gets long.

AM radio and shortwave radio frequencies are much lower, so the wavelength is much longer, and because the antenna needs to generally be at least 1/4 of the wavelength, the antenna must be much longer. So, antennas for AM broadcast or shortwave stations are so big that the entire tower IS THE ANTENNA. They're generally fed at the bottom end which has high impedance, yet the transmission line is low impedance to reduce losses and an impedance matching network must be used. That network is called an antenna tuner or Antenna Tuning Unit (ATU). And like the antenna, as the wavelength gets longer, the parts (inductors, capacitors, etc.) must also get bigger. (Yes, the foregoing is all provided to make my long story even longer. )

So, the photo is showing the ATU for a powerful, low frequency transmitter. The entire room is the ATU and the two guys inside are likely making fine adjustments to "tweak" the tuning of the impedance matching network. You can see the inductors that are the long, round coils.

I would have thought that personnel would only be allowed in such a tuner when the station is off the air, but MJK explained that my suspicion wasn't necessarily true and that personnel will sometimes go into such a room to make adjustments while the station is broadcasting. From a practical standpoint, the appeal of adjusting a live transmitter is obvious: one can immediately see the results of the tuning by making adjustments while watching the meters. But at these power levels, it is not without serious risk. So, as MJK stated,"that ATU is a death chamber for certain". Or paraphrasing the Klingons, it is a good place to die.

Similarly, there are transmitter rooms with the final output tubes (huge tubes) that are fed with very high voltages. On a 1.5KW Ham transmitter, the plate voltage on the final amplifier often runs between 1,400 - 2,000 volts. At a large AM broadcast station, it can run in excess of 10,000 volts, perhaps more depending on the tube. So while a radio transmitter sitting on a desk is just a metal box, perhaps a foot wide, a broadcast station transmitter, like its ATU, can be an entire room, even a very large room.

So, your expression of being impressed was well founded. I shared the photo because I think this stuff is very cool - glad you liked it!

 

[Peter Batah]

@skier Can't believe I actually read your entire post. Mainly out of respect for the effort that you put into writing it. Still find it very interesting though. Perhaps, I should take a crash course os some sort. Radio Transmission 101. Or better still 0000001

 

[skier]

Kind of you to read it just because of my effort, though I reserve the right to believe (or delude myself) that you might have actually liked reading about it. I just never know how much someone would like to know. But because I personally like it when more info is provided, I try to do that in my own explanations, with the belief that those uninterested will abandon my prose when they've had enough (or too much), and the hope that I've given enough substance for other readers to learn what they wanted, or even better, enough with which to conduct their own searches if further interested. It's a crap shoot, so I throw the dice.

 

[skier]

Perhaps, I should take a crash course os some sort. Radio Transmission 101. Or better still 0000001

HA! Perhaps you didn't realize it, but you're already enrolled - that post WAS Radio Transmission 0000001.

 

[Peter Batah]

@skier No matter how hard I try I can't get away from zeroes and ones. You said; "enough with which to conduct their own searches.."

Well, that's what scares me. With Cubase and the DM I have plenty of searching to do as it is. One more interest is the last thing that I need right now.

And, by the way, most of what you wrote may have gone straight over my head. But, I still found it very interesting.

 

[-mjk-]

Jerry, a couple of AM broadcast stations that I worked on had those old RCA Ampliphase transmitters, that would take up one whole side of the room. You could heat your home with one of those things. 50kW and it sucked power out of the wall like a bad dream. That model was basically 2 complete 25 kW transmitters running in parallel and combined to generate 50kW. They were difficult to tune.

One station with one of those RCA transmitters in RI even had the old CONALRAD Civil Defense broadcast system. That system required outboard matching networks both in the transmitter and at each ATU at the base of each tower so the whole system could change to the CONALRAD frequency when activated. It had been taken out of service of course, but the guts were still there.

Eventually that station installed a Nautel solid-state 50kW transmitter. That rig was comprised of many individual modules so the transmitter would have to suffer a catastrophic failure for it to go fully offline.

I was the chief engineer of the local AM in my town. They had a beautiful BE digital transmitter when I took over the gig. Grady had installed it and he introduced me to the owner, who hired me to maintain the radio station, studio and transmitter site.

The transmitter was beautiful. It had 5 power levels, which was actually very useful for us (not many facilities need that many power levels). We were 1Kw day, and 76 watts at night. The FCC had actually provided the owner with post sunset and twilight power settings - 5 in total from Day to Night. I programmed them into the remote control to take advantage of the maximum amount of allowed power over time. With the old transmitter, as soon as the time advanced to the point where 1kW was over the limit, we immediately went to the nighttime power level because the old transmitter only had 2 power levels.

Interestingly, Grady had developed his famous Optimod modifications by that time, and we were running something like 140% modulation at night. Even though the carrier was only 76 watts, there was a lot of sideband power (and the intelligibility is in the sidebands) and that really made the station louder. During one FCC inspection, they said absolutely nothing about the modulation. The inspector was only interested in carrier power.

We had a grounded, folded monopole antenna. It was a 3-wire with a wire hoop skirt and I got nervous during every sleeting rain storm. But nothing bad ever happened to the wires. The transmitter site was in a wetlands and the Saugas river level went up and down seasonally. So, once a month I would go to the site and touch up the match because the driving impedance of the tower went up and down with the water level, lol.

I had nothing to do with the tower install. But, I knew the engineer who oversaw it, (another brilliant broadcast engineer) Dana Puopolo:

(the meat of the discussion begins at 19:40)

Dana said that the 3 anchors were not evenly spaced. He called the factory and the engineer asked him "Are you standing under it now?" with some concern in his voice. Apparently, one of the anchors was off by 5 degrees but nevertheless the tower was stable.

I've known Dana for almost as long as I've known Grady and Dana brought me in on some very cool projects.

My station was on 1360. There was a local 50kW on 680. Do the math. Driving down the highway past the 680, their second harmonic was so loud that you couldn't hear 1360.

 

[skier]

@-mjk- I really enjoy the stories of your past work! I didn't realize there are different available discrete power levels in some power amps. I thought you could adjust it with a vernier just like one can do on a Ham transmitter.

Here's a question, you can't modulate an AM signal more than 100% because you'll distort when it clips above 100%. But in FM, you're modulating the frequency; more modulation won't clip the signal, it just keeps increasing the spread of the signal. So doesn't that mean that when you've gone over the 100% point, you've increased the bandwidth you're using beyond that permitted by your license?

So far, I've watched about 30 minutes of the Dana video beyond the 19:40 point - fascinating stuff and I'm learning some things - I will watch it all.

Very funny about the changing water level at a station antenna yard changing the impedance match. I didn't think a wetlands commission would allow a tower to be installed in a wetlands area. I'd constantly be afraid the guy anchors were, well, not adequately anchored and could pull out. Even a free-standing tower in a wetland area sounds unsafe. A granite base - now that's something I could comfortably sleep under at night.

Wow! I'm surprised the FCC would license a nearby, powerful radio station having a frequency a major harmonic ( 2nd or 3rd) away.

 

[-mjk-]

hi Jerry. You ask some great questions. As for the modulation, in these examples we're talking about AM, and it's all about Peak Envelope Power. If your transmitter has the capacity to output the power level then yes, with the digital transmitters you can go something like 150% modulation. Most of today's AM stations are modulating over 100% to get good coverage. I remember working with Grady while he was doing modulation experiments and arriving at those conclusions. I've been fortunate to have been associated with he and Dana during those days. It blew my mind when they went beyond 100% modulation.

Dana is a trip. He's really down to earth. He loves music and he used to call me "The Mario Lanza of Rock and Roll" which was an incredible compliment. Dana is another very approachable engineer. You could very easily contact him and engage him in dialogue.

In the old days, a wetlands was the perfect place for a transmitter site and they were actually sought out as locations for that, and also because they are usually remote with no habitation around. These days it would be impossible. In the case of my station, only the tower was in the wet area. They had dug a considerable hole to pour the base and it was stable. But the ground around the base was mushy.

As for the nearby 680, it's all about who was there first. And we were not first, lol.

 

[skier]

I never realized that wetlands were allowed as locations for antenna towers because of their much lower stability than solid, dry ground. However, I understand the appeal: the more conductive the ground, the better it acts as a ground plane due to its better permittivity. So I presume these were tidal wetlands so they had some salinity resulting in the wetlands being at least somewhat brackish and therefore more conductive? I know that fresh water generally is a terrible conductor (unless it has a very high mineral content) and you need many more antenna radials for the other 1/4 wavelength (I need lamda - we need the ability to use Greek symbols for these types of discussions - as moderator, please make it happen ) or you get higher ground losses. Regardless, I've learned something new again - thank you!

Yes, Dana definitely comes across as an expert; everything I heard from him with which I'm familiar is correct, he's consistent in his technical assertions, and that influences me to believe the rest is also correct. I've learned over my life that vetting someone I'm hoping to learn from is very important if I want to avoid learning garbage. There are too many people who either assert bad info because they're just repeating something they heard that's wrong, but they didn't know it, or worse, some individuals make stuff up as they go along to sound important. In my teens, we had a rhythm guitarist who was a good musician, but he often spouted questionable or downright stupid assertions. We had a saying about him: "If you ask Steve a question and he knows the answer, he'll tell you. But if he doesn't know, he'll tell you anyway." Such a person does not engender confidence in trusting his responses.

The Mario Lanza of Rock and Roll

I love his nickname for you! Was what you had in common your singing abilities? Or your lifestyles? HA!

Yes, I'd also like to meet Dana some day. (Though, that's probably less likely after my preceding comment, huh?)

 

[-mjk-]

@skier said:

I love his nickname for you! Was what you had in common your singing abilities? Or your lifestyles? HA!

I've lived 27 years past his expiration date, so I hope it's not my lifestyle, lol.

You are also correct about the tidal aspect of that particular property. By the way, that transmitter site is now gone.

Dana is totally cool. He would agree with your previous statement.

The point you make about not knowing is a very good one. Both Dana and Grady freely and openly say "I don't know" when they don't know. If they go on to postulate they make sure you understand that it's postulation.

 

[skier]

I've lived 27 years past his expiration date, so I hope it's not my lifestyle, lol.

HA! I can see how you'd feel like that.

The point you make about not knowing is a very good one. Both Dana and Grady freely and openly say "I don't know" when they don't know. If they go on to postulate they make sure you understand that it's postulation.

I couldn't agree more. Honestly responding that we don't know something is something to respect - none of us even knows a lot, let alone everything.

 

[skier]

I forgot to say, I did watch the rest of the Dana/Kurt interview and enjoyed it very much - thank you for providing that link! It's gratifying to hear experts in a field explain how things work and especially, sharing those valuable little hard-won insights into a complex field that often only come form vast experience.

 

[-mjk-]

What I like about those types of interviews is that Dana just gave away thousands of dollars of billable time by telling you how to do something that he would ordinarily be paid to implement.

I've worked with Dana on quite a number of projects. He's a good boss to work for. I gotta give him a call and see how he's doing.

Edit: since his name came up and I haven't talked with him in several years, I got his number from Grady and I've left a message for him. I'm glad his name came up in our conversation!