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Author Topic: Spring at the base of a 800 MHz antenna  (Read 344 times)
KB9SAY
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Posts: 22




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« on: January 22, 2003, 11:27:41 PM »

I have been using a small spring that (3/8 inch OD) at the base of my mobile antenna that works fine at VHF and UHF frequencies. For some reason at 800 MHz and above it changes the resonance frequency, you can just touch the antenna spring lightly and the center frequency can shift as much as 100 MHz. It asks like an intermittent problem. Has any one seen a similar problem and have a solution? I need flexibility at the antenna base. I have been using an 8753 network analyzer to do my measurements.
73s
KB9SAY
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AB0TJ
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Posts: 76




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« Reply #1 on: January 22, 2003, 11:52:41 PM »

Probably because this small spring 'looks' small enough (in terms of wavelength) to a VHF signal that it does not make much of a difference, but at higher frequencies it can make the antenna change length a bigger percent of the wavelength than it would at VHF because 800 MHz has such a shorter wavelength. When using springs at the base of the antenna like that, the spring acts as part of the antenna. Also the resonance changes when you touch it for the same reason, your body is interacting and becoming part of the antenna.

Of course, this is all assuming I understand your post correctly... I get the picture of mount->spring->antenna

73 and GL,
Alex AB0TJ
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AC5E
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Posts: 3585




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« Reply #2 on: January 23, 2003, 07:43:46 AM »

Hi: most "springs" have a piece of braid up the middle, in parallel with the coil itself. This is intended to maintain a continuous low impedance RF path no matter how wind resistance and vehicle motion causes the spring to flex.

I have seen the braid break, causing an unpleasant intermittant at HF. I have also seen an uninsulated braid make contact with the coil, shifting the resonance point of the antenna system.

On 2M and even more so on 70 CM the braid has considerable inductance by itself, and if even a fraction of a turn of the coil fails to make complete contact with the coil above and below it you have another inductance in parallel. That's enough to cause problems on VHF, where manufacturers generally advise you to not use a spring of any sort.

On 800 mHz - I don't think I would use a spring under any circumstances.

73  Pete Allen  AC5E
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K5DVW
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Posts: 2193




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« Reply #3 on: January 23, 2003, 07:44:36 AM »

That spring looks like a large inductor at 800Mhz. It probably looks intermitant because some of the spring turns are shorting due to oxidation or dirt or something. When the turns are shorted, the inductance is lower.

At VHF, the spring doesnt have enough reactance compared to the antenna to notice this type of behavior.
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KB9SAY
Member

Posts: 22




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« Reply #4 on: January 23, 2003, 08:53:59 AM »

I think I understand what is going on here. There is a braid inside the spring. If the spring is completely closed and makes good contact from coil to coil, all the RF remains on the out side of the spring due to skin effect. It will act more like a large diameter conductor. The braid does nothing at this time because there is no RF inside. If there is flexure in the spring or intermittent contact in-between the coils (from dirt, oil, and so on) the spring becomes more of an inductor. The braid may now see RF and be in parallel with the spring’s inductance. At HF and VHF this may not cause much of a problem unless of coarse the braid breaks as mentioned by AC5E. At 800 MHz and above the wavelength is much smaller and the flexure and intermittent contact in-between the coil have a greater effect on the resonance.
I’m not going to use a spring. I might try a high memory wire like a nickel titanium alloy or similar.
Thanks for all the help.
73s
KB9SAY
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W0FM
Member

Posts: 2056




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« Reply #5 on: January 23, 2003, 10:53:34 AM »

You've got it right.  On commercial mobile antennas, we don't use a spring on anything above 512 MHz.  800 and 900 MHz antennas have radiators that attach directly to the center of the mount, for the very reason you've deducted.

73 de Terry, WØFM
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