What is the typical complex Z for a 1/4 wave vertical ?

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Fran3
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What is the typical complex Z for a 1/4 wave vertical ?

Post by Fran3 » Thu Nov 19, 2015 5:54 pm

What is the typical complex impedance of a 1/4 wave tower with 120 1/4 wave radials? The tower is sitting on an insulator and insulator is sitting on a concrete base about 3 feet off the ground.

As I understand it Z for such a radiator is somewhere in the 39 ohms (right?) but what is the complex impedance typically?

R = what
j = what

Reason for asking is we are trying to characterize the ATU(s) with an antenna analyzer... but there is so much RF on the tower terminals due to two near by stations that we are concerned about damaging the analyzer if we connect it... So, we want to model the antenna and connect the model to the output of the ATU(s) and then perform measurements and trouble shoot.

I've suggested just using a 39 ohm resistor to trouble shoot inconsistent measurments as we switch the ATU from day configuration to night... but wondered if we could build an RLC circuit that would more closely look like the actual antenna.

So the questions are...

1 - What is the "ideal" complex impedance of a quarter wave vertical antenna and what will cause it to vary? Ground conductivity? Condition of ground radials? What?

2 - How the heck do you guys get these measurements when near multiple stations and have a hot tower?

Thanks for any comments or suggestions.

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Xenirad
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Re: What is the typical complex Z for a 1/4 wave vertical ?

Post by Xenirad » Thu Nov 19, 2015 8:24 pm

The theoretical complex impedance for a 1/4 wave vertical radiator should be around 37 Ohms +60j this of course can vary due to ground conductivity in the area and the current condition of the ground system.

Hope this helps.
Fred Francis, Owner Xenirad Broadcast Engineering http://www.xenirad.com

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Deep Thought
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Re: What is the typical complex Z for a 1/4 wave vertical ?

Post by Deep Thought » Thu Nov 19, 2015 9:21 pm

And the tower face width...and the guy wire insulator spacing...and anything else on the tower...and...
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Xenirad
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Re: What is the typical complex Z for a 1/4 wave vertical ?

Post by Xenirad » Thu Nov 19, 2015 10:33 pm

Deep Thought wrote:And the tower face width...and the guy wire insulator spacing...and anything else on the tower...and...
You are correct.
Fred Francis, Owner Xenirad Broadcast Engineering http://www.xenirad.com

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Shane
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Re: complex Z for a 1/4 wave vertical ?

Post by Shane » Thu Nov 19, 2015 10:52 pm

Fran3 wrote:2 - How the heck do you guys get these measurements when near multiple stations and have a hot tower?
We don't. OR we ask the station (in our case just one) if we can arrange some off time for us to do whatever work we are doing. How that would fly with multiple stations I don't know. In any event, it's going to be overnight.

In our case, with our transmitter off, we can still draw an arc off the tower. The other station is about a mile and 70kHz away @ 5kw.
Mike Shane, CBRE
---Omaha---

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kkiddkkidd
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Re: What is the typical complex Z for a 1/4 wave vertical ?

Post by kkiddkkidd » Fri Nov 20, 2015 9:24 am

Jim Tonne's "Towers" program will give you a rough idea of what to expect from a series fed antenna in different configurations of height and diameter.

He also has a number of other utilities that are pretty handy to have around.

Later,
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Kevin C. Kidd CSRE/AMD
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http://www.amgroundsystems.com
KK Broadcast Engineering
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Re: What is the typical complex Z for a 1/4 wave vertical ?

Post by radio_guru » Fri Nov 20, 2015 10:04 pm

If you have a real network analyzer with a power amp and the proper directional couplers, you can overcome the ambient RF. We did a 6 tower DA with a Class A 50KW only a couple miles distant. On site field was something north of 1.5V/M. Same thing for doing tune-ups in the Meadowlands where no one shuts down except for an act of god.

One other thing to consider...IM products. If you're the higher frequency, you have a good chance it will be your problem to notch out their carrier to meet the mask. If that is in fact the case, a simple ATU is history. Been there done that, have the big ATU cabinet to bear that out. Mine was a -80Khz placing a 200mV field at my site. The SX series TX (New Nautel didn't improve the turn around loss either) needed 40dB of notch on that lower channel along with another 15dB to attenuate the image so as to meet the mask. Yet the station can still do IBOC....not that it matters much at this point. But the station sure does sound good. The consultants fees for that design and modeling effort were well worth the payments....

RG

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