[TowerTalk] Multi-Band Verticals

Jim Reid jimr.reid at verizon.net
Wed May 7 13:01:46 EDT 2003

Re,  the Fluidmotion Vertical Antennas,  BigIR,  etc.:

> What is the cost?

Price and info at:




> How does it compare to the screwdriver type ( continuously, remotely
> controled inductor)?

Not sure it does;  in the BigIR the radiating element is remotely
adjusted in length to resonate at any frequency between
7 MHz and 50 or so MHz,  including sw broadcast bands, WARC,
etc.  No remote inductors or other "hoo haw"'s,  hi.

Read the manual,  .pdf at:


> The screwdriver appears to not need so many radials as it is used
> mobile, is this so?

As with any ground mounted, 1/4 wave vertical,  the more radials
the better;  but over 30 or so,  diminishing returns are realized,
unless you must have the last dB or so,  hi.

Elevated,  pretty high,  then only two,  down sloping radials per
band are said to be sufficient;  but they interact and are a real
trouble to get all set up properly.  And if the ends of the radials
are within about 1/8th wave of ground,  you will need a ground
radial field anyway to terminate the powerful E fields which
develop at the high Z ends of those radials.

I believe vertical antennas intended to operated below 10 MHz,
such as the BigIR,  are best mounted either on the ground,  or
in my case up on the roof of the home if there is plenty of
room up there to lay out a radial field of at least 20 or more
wires;  such a set up will bring you within 65 to 75% or so of
the efficiency that would come about from 100 radials!  That's
within a couple dB of "perfection".

I enjoy mine,  and BTW,  use BeaconSee and this antenna
everyday to monitor the No. Cal. beacons,  all 18 of them
on 5 bands from 20 up to 10 meters;  very informative set
of plots.  BeaconSee will record the signals from each beacon
continuously,  or at sample times you may select,  from every
quarter hour,  up to as great an interval as a reading from
each beacon,  only every 3 hours on any or all of the bands
you select.  Very useful out here near mid-Pacific.

Hope this info answers your questions.

73,  Jim  KH7M

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