The antenna you've installed is not a base fed, ground mounted quarter wave
vertical. Your antenna length is measured from ground level (the bottom of
your 5 foot pipe) to the top of the vertical. Your antenna is actually five
feet longer than a quarter wavelength.
Your feed system is known as an elevated feed, which has higher
impedance than a base fed quarter wave vertical. Of course, the lack of a
radial system also increases your feedpoint impedance. Your 100 ohm feed
impedance is not surprising on the higher bands.
What band did you design this antenna for? The effect of 5 foot elevated feed
on feedpoint impedance is small on 160 meters, but much more significant on the
---- Original message ----
>Date: Thu, 10 Jul 2008 19:39:16 EDT
>Subject: [TowerTalk] Ground Mounted Vertical Question
>I'm installing a full-size ground mounted 1/4 wave vertical. I mounted the
>vertical on a 5' pipe. The vertical is insulated from the mast pipe using a
>base from an old Hustler 4BTV vertical. I will now get to work on the ground
>Question: With the full-size ground mounted vertical connected only to the
>5' mounting pipe, no radials, I thought I'd see what the SWR and impedance
> I was expecting a low impedance of about 10 ohms. (poor ground = low
>impedance = low performance)
>However, using a MFJ-259B at the input of the antenna the impedance, at
>resonant freq. is 100 ohms. (SWR over 2:1 as would be expected at 100 ohms).
>I'll install the ground radials next... but the above doesn't seem right.
>All thoughts appreciated!
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