ATU rebuild

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grich
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ATU rebuild

Post by grich » Mon Nov 05, 2018 8:48 pm

One of my current projects is to replace a rotted-out shack on a ND 1KW site.

The existing network is spread out linearly on a shelf, it's a pi network with an extra series inductor on the input and a capacitor in the output line, presumably to cancel Xl of the tower. The licensed R is 425 ohms, but there are no measurements on file showing the current tower layout, with just a bare tower and tower lights. I have sweeps going back to 1966 when an FM antenna with deicers and a Marti antenna were all hung on this tower. All of those sweeps set the R in the 400's and +j200-ish.

Last time I actually bridged a tower was 30 years ago with my mentor, Warren Stucki, who passed away at age 96 over a week ago. We had a GR916 and an HP signal gen the size of a Smart car. This time around, I wish I had a VNA, but I have an OIB-1, so I hung it on the tower over the weekend and measured 355 ohms resistance and a reactance of +j281.

The rotten shack is going to be replaced with a classic Harris cabinet with one of the dreaded big single coils. I ran T-network calculations with the RFS calculator and it's estimating 13.95 uH on the input leg, 786pF on the shunt leg, and 709pF on the output leg. Almost sounds doable. Thoughts?

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Dale H. Cook
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Re: ATU rebuild

Post by Dale H. Cook » Tue Nov 06, 2018 4:42 am

Have you done a Smith chart of the proposed design to see what the cusps look like, and to see if they need rotation?
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

grich
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Re: ATU rebuild

Post by grich » Tue Nov 06, 2018 11:27 am

No, and that will have to wait until I recover from Election night broadcasts. :D

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Dale H. Cook
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Re: ATU rebuild

Post by Dale H. Cook » Tue Nov 06, 2018 12:16 pm

To do a Smith Chart you need to use a signal generator and detector with the OIB to measure the base impedance at, say, 1kHz increments out to +/- 10 kHz of the carrier frequency in order to look at the cusps.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

rfn
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Re: ATU rebuild

Post by rfn » Tue Nov 06, 2018 4:55 pm

It sounds like a project that won't be quickly finished.

If, during the course of it, you find a need for vacuum caps I might be able to help. I have a huge assortment of various values in storage, any or all of which are available pretty much for any reasonable offer. Shipping could be costly as they're all in Nome, Alaska. I don't have a listing of their values but probably will be there later this month so if you think you might need some let me know what and I'll look through them.

There's also a wealth of Nautel ND series parts - I'll try to remember where to "list" them on this board....it's been years since I've done any of that.

I check in pretty much daily so PM communication should work OK.

ChuckG
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Re: ATU rebuild

Post by ChuckG » Tue Nov 06, 2018 9:13 pm

Are you using an adjustable element in the shunt leg? Those old Gates single-coil T-networks with a fixed shunt cap were less than fun to make look good off-carrier.
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Deep Thought
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Re: ATU rebuild

Post by Deep Thought » Wed Nov 07, 2018 10:34 am

DO NOT--repeat--DO NOT try to do this with a "single coil" network. It will be unstable as hell, almost impossible to tune due to the stray reactances and cross coupling between the turns on the coil, and will have shit for bandwidth.

I will be happy to send you a proper design for free just to save you from the torture you are about to endure.

An OIB-1 is not going to give you an accurate tower self resistance for a tower with that high an R and X. Design to the last measured tower impedance unless you have a reason to think it has changed.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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RGORJANCE
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Re: ATU rebuild

Post by RGORJANCE » Wed Nov 07, 2018 11:52 am

Ahhh, yessss! The infamous "Phantom Tee".

As stated by others----avoid that at all costs! You would be better off winding your own coils out of 1/4" copper tubing around a stiff form like PVC. Don't use the home made coil with PVC still in it.

Fossil

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Dale H. Cook
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Re: ATU rebuild

Post by Dale H. Cook » Wed Nov 07, 2018 2:58 pm

Deep Thought wrote:
Wed Nov 07, 2018 10:34 am
An OIB-1 is not going to give you an accurate tower self resistance for a tower with that high an R and X. Design to the last measured tower impedance unless you have a reason to think it has changed.
If you have the last measured impedance I hope it was a full sweep, which will let you optimize the bandwidth. If feasible I suggest a new sweep with a GR 1606 or 916 bridge. There are reasons why some of us keep those in our arsenal.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

Steve Brown
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Re: ATU rebuild

Post by Steve Brown » Wed Nov 07, 2018 6:38 pm

Greg, if this rebuild is for KSIB-AM, then I was around when the 'new' ATU got built. I think that it happened when the FM Class A went on the air. The carpenters at the Farmer's Coop built the doghouse and Harold Bauman, the engineer at the time, moved the ATU components from the old decrepit metal cabinet to the new shed. I was happily working 7 days a week at the station at the time, and I don't recall anyone bridging anything, which I would have had a great interest in at the time. For some dumb reason I remember the base current was 1.59 amps, so I have the feeling that the components got moved and the taps tweaked to produce that value while keeping the line current meter in the old Gates BC1-T reading the same as before.

Steve Brown Radio Rangers, Mpls

grich
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Re: ATU rebuild

Post by grich » Wed Nov 07, 2018 9:18 pm

Deep Thought wrote:
Wed Nov 07, 2018 10:34 am
DO NOT--repeat--DO NOT try to do this with a "single coil" network. It will be unstable as hell, almost impossible to tune due to the stray reactances and cross coupling between the turns on the coil, and will have shit for bandwidth...
RGORJANCE wrote:
Wed Nov 07, 2018 11:52 am
Ahhh, yessss! The infamous "Phantom Tee".

As stated by others----avoid that at all costs!...
I had read a few opinions elsewhere of the single-coil ATU, and they weren't favorable. The comments of the members of this august forum are appreciated and will be heeded. Deep, I may take you up on your generous offer, as I have torture aplenty right now...a buck just totalled my 2-year-old Jeep. I have the cabinet sitting on the bench with the expectation that I would be gutting the box and drilling mounting holes for a new configuration. Perhaps I can make the big coil into some art for the reception area. :lol:

grich
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Re: ATU rebuild

Post by grich » Wed Nov 07, 2018 9:32 pm

Dale H. Cook wrote:
Wed Nov 07, 2018 2:58 pm
Deep Thought wrote:
Wed Nov 07, 2018 10:34 am
An OIB-1 is not going to give you an accurate tower self resistance for a tower with that high an R and X. Design to the last measured tower impedance unless you have a reason to think it has changed.
If you have the last measured impedance I hope it was a full sweep, which will let you optimize the bandwidth. If feasible I suggest a new sweep with a GR 1606 or 916 bridge. There are reasons why some of us keep those in our arsenal.
Unfortunately, if a sweep was done when the current licensed impedance was established, that documentation is missing. I know someone with a 1606 and a PowerAim, and I think I can enlist his help in getting a proper adjustment of what gets built. We definitely want good bandwidth.
Last edited by grich on Wed Nov 07, 2018 9:55 pm, edited 1 time in total.

grich
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Re: ATU rebuild

Post by grich » Wed Nov 07, 2018 9:55 pm

Steve Brown wrote:
Wed Nov 07, 2018 6:38 pm
Greg, if this rebuild is for KSIB-AM, then I was around when the 'new' ATU got built. I think that it happened when the FM Class A went on the air. The carpenters at the Farmer's Coop built the doghouse and Harold Bauman, the engineer at the time, moved the ATU components from the old decrepit metal cabinet to the new shed. I was happily working 7 days a week at the station at the time, and I don't recall anyone bridging anything, which I would have had a great interest in at the time. For some dumb reason I remember the base current was 1.59 amps, so I have the feeling that the components got moved and the taps tweaked to produce that value while keeping the line current meter in the old Gates BC1-T reading the same as before.

Steve Brown Radio Rangers, Mpls
Darn...busted! :D

So that shack is 50+ years old...I have the 1966 tower sweep done by John Heffelfinger of Kansas City, which of course does me little good today.
The current ATU components sure look like they came from the original 1946 installation, except for a shunt cap replaced after a lightning hit before my time. I wager the antenna match became a bit more critical when the BC1-T was replaced with the MW-1 around 1980.

The present base current is 1.53 amps, so your memory's pretty good...might have been 1.59 with all of the other junk across the tower base back then.

COMMENG
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Re: ATU rebuild

Post by COMMENG » Thu Nov 08, 2018 6:46 pm

grich wrote:
Mon Nov 05, 2018 8:48 pm
One of my current projects is to replace a rotted-out shack on a ND 1KW site.

...This time around, I wish I had a VNA, but I have an OIB-1, so I hung it on the tower over the weekend and measured 355 ohms resistance and a reactance of +j281.

The rotten shack is going to be replaced with a classic Harris cabinet with one of the dreaded big single coils. I ran T-network calculations with the RFS calculator and it's estimating 13.95 uH on the input leg, 786pF on the shunt leg, and 709pF on the output leg. Almost sounds doable. Thoughts?
I don't know what the frequency is but I ran my T-match software for 90 degrees phase shift and for 1.5 MHz and 355R which shows a shunt cap of 794 pF and 14.13 uH for each series leg.

I would rebuild for a T-match and keep the Harris single coil system for some other rebuild.


COMMENG

Steve Brown
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Re: ATU rebuild

Post by Steve Brown » Thu Nov 08, 2018 6:57 pm

I happen to know that Deep has some inductors that might work for you---whine and beg at him.
Steve B

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