Unipole antenna resonance

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WALKINGMAN
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Joined: Thu Jul 16, 2015 12:49 am

Unipole antenna resonance

Post by WALKINGMAN » Thu Jul 16, 2015 1:17 am

This is my first time posting please bear with my english,

I have a problem with my AM antenna resonance. When I measure my antenna impedance the reading is 73 ohms + j108.69 ohms.
Im using an analyzer and set to reflection mode my antenna resonance is 871.5 khz . My operating frequency is 999 khz. So I tuned my ATU to match the 50 ohm trnamsission line, but my ATU tee network reactive output arm is coil. In order to match the atu I need to add a capacitor in series with output coil. the problem is I dont have capacitor avaiaible and I want to use the coil only. My plan is to re configure my antenna six drop wires the existing antenna uses 1/0 stranded wires if I change to 3 drop wires is my resonance will go up or to reduced away from 999 khz? When I measure the ATU in put the pattern/ bandwidth became narrow. I dont have program tools to verify if I will change my drop wires spacing, size of wires, number of wires will increase my resonance and also the reactive will reduce and my antenna will now became inductive instead of capacitive . Doing it in try and error is very hard labor and time consuming. I dont want to tamper the circuit of ATU I want to fix my antenna to 3 drop wires instead of six wires.

Hope I have some input from the resident expert Engineer.

Regards,

tony

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Deep Thought
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Re: Unipole antenna resonance

Post by Deep Thought » Fri Jul 17, 2015 10:24 am

Welcome to the site, Tony. Your English is just fine. :mrgreen:

Unfortunately, there is no way to do what you want. The unipole skirt is inherently inductively reactive and you need the series capacitor to have any chance of getting this to j0. You'd be much better off trying to come up with a tee network that would use the reactance as part of the output leg, but even then you'd probably need a capacitor in the output leg. However, you say you want to avoid messing with the existing network, which is also not going to work.

Did you add this skirt to an existing series-fed tower?

If you can give me the height of the tower I can play with this a bit to at least give you some options with spacing, number of folds, etc. Maybe one of them will allow your ATU network to match this without additional parts.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

WALKINGMAN
Posts: 11
Joined: Thu Jul 16, 2015 12:49 am

Re: Unipole antenna resonance

Post by WALKINGMAN » Fri Jul 17, 2015 8:26 pm

Hi Deep Thought

Thank you for the reply, my tower height is 250 ft, it is a series fed before , but it was change already after the original tower was toppled by super typhoon. I ilke to convert the existing 6 drop wires into six wires antenna beleiving it will transform my antenna resonance near to my operating frequency, I also thinking that if it resoanated to 999 khz itwill radiate stronger FI than leaving its resonance to 871.5 khz and match into ATU.


Tony

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Deep Thought
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Re: Unipole antenna resonance

Post by Deep Thought » Fri Jul 17, 2015 10:46 pm

Resonance means nothing if you have a tuning network. Don't get preoccupied with that. Your primary goal is maximum power transfer and minimum power loss.

Using a 250 foot tower height and a 24" face triangular uniform-cross-section guyed tower, six folds and 24" spacing from the tower I get this:
55a98f0b.jpg
The 50 ohm bond point is:
55a98e85.jpg
I don't think you'll be able to get the +j down any lower than that, but a Tee network to match this at the 50 ohm R point is pretty easy. You'll still need the series capacitor on the output leg though.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

WALKINGMAN
Posts: 11
Joined: Thu Jul 16, 2015 12:49 am

Re: Unipole antenna resonance

Post by WALKINGMAN » Sat Jul 18, 2015 7:57 am

Thanks

Your data will help me a lot in analyzing my antenna's problem, probably my radials is the culprit why i have hi impedance and j despite of my adjustment of the shorting stub.


Tony

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