950 MHz STL Dish on AM Tower

AM Radio discussion. Directional arrays are FUN!
User avatar
kkiddkkidd
Posts: 487
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: 950 MHz STL Dish on AM Tower

Post by kkiddkkidd » Fri Aug 26, 2016 6:14 pm

ChuckG wrote:The paramedics would then bring a bucket truck anyway.
Not to mention a dozen pairs of hemostats and needle nosed pliers to yank out the splinters after you cut out and hug the pole as gravity does it's thing...

If you see my fat, bald butt climbing a pole, you probably want to join me up there.
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

User avatar
NECRAT
Site Admin
Posts: 2924
Joined: Sat Nov 17, 2007 9:13 pm
Location: Taunton, MA
Contact:

Re: 950 MHz STL Dish on AM Tower

Post by NECRAT » Fri Aug 26, 2016 7:24 pm

awsherrill wrote:This all happened over eight months ago according to the date stamps. Wonder if the OP has resolved the problem by now.
The OP hasn't logged in since February. This thread is old, and dead.
http://www.necrat.us

"Arguing with an engineer is like mud wrestling with a pig. After a couple of hours, you realize the pig likes it"

BigRed
Posts: 85
Joined: Tue Mar 27, 2012 8:59 am

Re: 950 MHz STL Dish on AM Tower

Post by BigRed » Fri Aug 26, 2016 9:56 pm

Just because the OP hasn't logged in since February it doesn't mean that he's not been reading the thread.

And a suggestion . . .

Periscope. If the STL is a receive only then a modified periscope could be used with very minimal impact on the AM situation, depending on the link budget. (The FCC frowns on periscopic antennas at the transmit end these days but I'm pretty sure that you can do it at the receive end of a link.) And the cleanest way to do that is to place the STL antenna on the tower and run the feedline down to almost the base. Place another antenna there aimed down towards the ground (a log-perodic will probably do fine at 950-mHz, again depending on the link budget). And then set up another antenna a close distance away from that antenna with its feed line continuing on to the STL receiver. That gap between the two antennas at the base of the tower will get the STL signal across the tower base without disrupting the base impedance. Just be sure to bond the transmission line shield to the tower at appropriate intervals. Also, be aware that the actual receive antenna near the top of the tower may cause a very minimal amount of "top loading". That top loading probably won't be noticed except in the most critical situations though.

As to wooden poles . . . I once had to do a 90-foot pole for a 2-gHz ICR link, it's still standing 73-feet in the air at the site after 20-years and still supporting an Andrew 8-foot grid antenna. It's "charm" was that it was cheap to plant, required no foundation (PE stamp). But for the first half of its life it required the antenna be re-aimed at the beginning of every season because as the wood continued to dry the pole would twist and turn slightly, just enough to mis-align the path. And as others pointed out even though the pole itself is non-conductive there's still the ground wire (you did order the pole with that wire already attached, along with the copper bottom plate?) and the coax run from the STL receive antenna. So de-tuning that structure will still be necessary.

User avatar
Deep Thought
Posts: 3160
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: 950 MHz STL Dish on AM Tower

Post by Deep Thought » Fri Aug 26, 2016 10:56 pm

BigRed wrote:So de-tuning that structure will still be necessary.
Happily, that is very easy to do. Interrupt the ground wire and wrap about 40 turns of #8 wire around the pole to form a coil. DC continuity is maintained but RF won't bother to try to get through. You can also resonate it with a capacitor if necessary.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

User avatar
Shane
Posts: 727
Joined: Fri Feb 01, 2008 12:08 am
Location: Omaha
Contact:

Re: 950 MHz STL Dish on AM Tower

Post by Shane » Sun Aug 28, 2016 2:04 pm

Thanks for not closing the thread, Mike. I for one learned very valuable information from both the earlier and later posts.

Sometimes when someone reactivates a thread I find older posts that I had missed. Possibly because the site logged me out before I could get thru all my new posts. That's one annoyance I have with this board. It assumes you read all the new posts if you log out, purposely or not.
Mike Shane, CBRE
---Omaha---

BigRed
Posts: 85
Joined: Tue Mar 27, 2012 8:59 am

Re: 950 MHz STL Dish on AM Tower

Post by BigRed » Sun Aug 28, 2016 9:24 pm

Deep Thought wrote:
BigRed wrote:So de-tuning that structure will still be necessary.
Happily, that is very easy to do. Interrupt the ground wire and wrap about 40 turns of #8 wire around the pole to form a coil. DC continuity is maintained but RF won't bother to try to get through. You can also resonate it with a capacitor if necessary.
That coil in the ground wire near the base of the pole is a neat idea but I think that you've overlooked a couple of things . . .

1st, if the pole is supporting a communications (STL) antenna you'll have to also add an isolation coil for that feedline as well. A sample loop isolation coil should work just fine for that.

2nd, while that approach will be fine and dandy for DC and power line frequencies it will have the same effect for a lightning strike discharge as it does for the AM band. (The impulse frequency of a lightning strike is somewhere in the middle of the AM band, or so I'm told . . . ) So either a ball gap should be included across the coil or, perhaps, forgo the coil in the ground wire and use a single de-tuning skirt wire with coil/capacitor so that the ability of that ground wire to pass a lightning discharge to ground is not compromised.

User avatar
Deep Thought
Posts: 3160
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: 950 MHz STL Dish on AM Tower

Post by Deep Thought » Sun Aug 28, 2016 11:06 pm

You can wrap the STL coax around the pole, too.
So either a ball gap should be included across the coil
Usually I'll have the power co. put one of their lightning arrestors across the coil. I've got a photo of such an installation around here somewhere.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

Post Reply