How do the AM pros do this?

AM Radio discussion. Directional arrays are FUN!
Post Reply
cottagefarmer
Posts: 7
Joined: Thu Mar 13, 2014 5:24 am

How do the AM pros do this?

Post by cottagefarmer »

Hey fellers,

As a ham I have a 58' aluminum tower (lightweight, about 14" each side of triangle). It is a base-insulated tower with a corona ring at the top. It's used for 160, 80, 40 and 30 M.

I want to install a very small perforated dish, and the dish also has a 5 GHz Transceiver, powered with Power over Ethernet for AREDN on it. How do the professionals electrically isolate such a peripheral? Should I worry about the Cat 6 cable? How should I deal with protecting the HF bands and how to also electrically protect the 5 GHz transceiver? Then at the bottom how would you route the cable?

Thanks guys,
Patrick
User avatar
Dale H. Cook
Posts: 937
Joined: Thu Dec 20, 2007 9:08 am
Location: Roanoke/Lynchburg, VA
Contact:

Re: How do the AM pros do this?

Post by Dale H. Cook »

cottagefarmer wrote: Sun Dec 08, 2019 10:42 pmHow do the professionals electrically isolate such a peripheral?
We use a device called an isocoupler:

https://www.kintronic.com/product-categ ... socoupler/
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html
ncradioeng
Posts: 79
Joined: Wed Nov 18, 2015 3:58 pm

Re: How do the AM pros do this?

Post by ncradioeng »

Wind your own isolation choke with some Cat5 similar to the lighting choke link below. Don't use the capacitors on each end of course. Two or three inch PVC should work for the form.

https://www.kintronic.com/shop/lighting-choke/
cottagefarmer
Posts: 7
Joined: Thu Mar 13, 2014 5:24 am

Re: How do the AM pros do this?

Post by cottagefarmer »

Ok great, thank you!
User avatar
kkiddkkidd
Posts: 947
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: How do the AM pros do this?

Post by kkiddkkidd »

As already mentioned, you have to isolate EVERYTHING that crosses the base insulator. There are only a few ways to effectively do that.

Use GOOD quality shielded Cat5 and shielded connectors on the tower. As a minimum, I use UBNT's Carrier grade shielded cable but have had to resort to using this (https://www.tessco.com/product/armor-ca ... ant-315167) cable and small universal ground kits every few feet along the cable.

1. Isolate the Cat5 using something like https://www.kintronic.com/wp-content/up ... utions.pdf. Big bucks...

2. Isolate AC voltage crossing the base insulator with something like https://www.kintronic.com/shop/lighting-choke/and use a 2nd wireless device to get the data across the insulator.

3. Similar to 2, Isolate AC voltage and use a pair of fiber transceivers to get the data across the insulator.

4. Convert to a shunt feed...

5. I think that you could probably wind enough shielded Cat5 on a form to effectively isolate it but have never tried.

Although I have never tried to wind one myself, a lighting choke such as above is nothing more than a LARGE inductance across the base insulator. IIRC, a lighting choke needs to be about 5000 ohms of inductive reactance to be effective. There are others on this forum that would be able to address that better than I.

You could certainly wind an AC isolation choke or a choke to isolate the POE power and then use a 2nd pair of radios or fiber to cross the insulator with data.

I just got thru installing a network link on an AM tower. This tower already had tower light wiring crossing the base insulator via a 3 circuit lighting choke (such as https://www.kintronic.com/shop/lighting-choke/). I moved the lighting photocell to the tower (the flasher was already mounted on the tower) and borrowed power from the lighting circuit to power the top UBNT RocketM5 radio for the main link and the base mounted UBNT NanoStationM5 for the tower to building link.

A few months ago I did the fiber thing using a pair of these (https://www.amazon.com/dp/B0716XT1QT/re ... 7Db5SWMYZB) fiber transceivers and about 20ft of appropriate fiber. This tower also had lighting power already isolated and on the tower.

I have also done the 2 radio link scenario when the top dish points somewhere close to the TX building by making the top dish an AP (not always possible in an existing network architecture), the distant unit a client and then simply add another client at the TX building pointing upward toward the AP dish. The off axis angle does limit thru put but it seems to work fine as long as the signal is good.

Both are working great. HOWEVER, sometimes there are situations that just refuse to work. Isolating across the insulator is pretty predictable but sometimes it is a real challenge to keep the network radios working with the AM TX on.

Let us know what you do and how it works.

Regards,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com
User avatar
Deep Thought
Posts: 3555
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: How do the AM pros do this?

Post by Deep Thought »

What is the base impedance at each frequency of interest? One rule of thumb is to design an isolation coil with at least 50 times the Z at the tower feedpoint, which means a short tower with a 20 ohm base impedance would require a +j1000 ohm coil, or around 88 μH at 1.8 MHz.

Most of the commercial units consist of copper tubing wound in a coil with cat5 or 6 inside, terminated on an RJ45 jack at each end. You could easily build one of these yourself: A 10" diameter coil with the windings equally spaced along a 24" coil length would require 32 turns, which would easily fit 3/8" ID refrigeration tubing carrying the cat6. A larger diameter (say, 20") would only require 17 turns. If you have the space (and the budget...copper tubing has gotten ridiculously expensive lately) you could wind a bigger coil to assure less "disturbance" but since this is a single vertical radiator as a practical matter it would be easier and wiser to adjust your antenna tuner to compensate. The 88 μH coils I mentioned above would require 84 and 89 feet of tubing, respectively. If you can find shielded cat5/6 with a stiff enough shield that can support the cable in a coil you can forego the tubing and just wind the cable on a form, but you need to make 100% sure you can get a solid ground connection on the "cold" end or you'll be in for a nasty surprise when you energize the tower.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com
User avatar
kkiddkkidd
Posts: 947
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: How do the AM pros do this?

Post by kkiddkkidd »

Deep Thought wrote: Mon Dec 09, 2019 11:34 am but you need to make 100% sure you can get a solid ground connection on the "cold" end or you'll be in for a nasty surprise when you energize the tower.
It will give a whole new meaning to the term "packet collision"...

The rodent resistant direct burial cat5 noted in my post is probably stiff enough to wind with two or 3 supports. It isn't a continuous copper jacket a la heliax, but it does have an overlap of the thick copper armor of about 1/4 turn (much like PE89). I want to experiment with isolation coils on Cat5 but there never seems to be enough time or daylight.

Every AM tower that I have mounted a network radio upon was either shunt fed or had power available above the base insulator so never had to get that creative.

Later,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com
User avatar
kkiddkkidd
Posts: 947
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: How do the AM pros do this?

Post by kkiddkkidd »

Another question that I thought of was.... What effect would the natural reactance of the tower have on the isolation coil as its j swings positive to negative as the bands change? Or does it matter?

Aside from the general AM aspect of this discussion, I know that as many IP radios as I am deploying that it will come up in the not too far off future.
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com
User avatar
Deep Thought
Posts: 3555
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: How do the AM pros do this?

Post by Deep Thought »

kkiddkkidd wrote: Mon Dec 09, 2019 4:08 pm Another question that I thought of was.... What effect would the natural reactance of the tower have on the isolation coil as its j swings positive to negative as the bands change? Or does it matter?

Aside from the general AM aspect of this discussion, I know that as many IP radios as I am deploying that it will come up in the not too far off future.
That's why I asked about the tower Z upthread. The coil reactance will increase as the operating frequency increases but you really don't want to find yourself in a nasty surprise parasitic resonance situation.

I've done a half dozen series-fed towers using the isocoil method...one was a KTL and one a Phasetek unit; the other four were homebrew. I also just did a MoM conversion on a DA where one tower had a LED "strobe" on it fed using eight conductors through a Phasetek coil. No idea why it took eight conductors but the installation is about the same as for cat5.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com
User avatar
kkiddkkidd
Posts: 947
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: How do the AM pros do this?

Post by kkiddkkidd »

Deep Thought wrote: Wed Dec 11, 2019 9:41 pm
That's why I asked about the tower Z upthread. The coil reactance will increase as the operating frequency increases but you really don't want to find yourself in a nasty surprise parasitic resonance situation.

I've done a half dozen series-fed towers using the isocoil method...one was a KTL and one a Phasetek unit; the other four were homebrew. I also just did a MoM conversion on a DA where one tower had a LED "strobe" on it fed using eight conductors through a Phasetek coil. No idea why it took eight conductors but the installation is about the same as for cat5.
I have installed several KTL isocoils for STL & xlator isolation and was very happy with the results. One set of 3 isocoils were replacing a combination of another brand coil type isolator AND another brand capacitive type isolator that had destroyed themselves several times. All of the existing isolators were low budget and the KTL isocoils fixed a number of problems they were having. It was believed that the combination of inductive and capacitive isolators were interacting and blowing each other up as well as allowing AM energy to damage the output devices in the FM translator transmitters.

Later,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com
dd92251
Posts: 45
Joined: Sat Mar 30, 2013 4:21 pm

Re: How do the AM pros do this?

Post by dd92251 »

Run your equipments coax thru about 10 Ferrite cores ( FT 50-43 ) at the point where it comes off the tower. Do this for each Line, power line, Cat-6, etc. See if that works.
Post Reply