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Author Topic: Antenna question  (Read 6089 times)

Posts: 486

« on: February 24, 2013, 11:00:46 PM »

I was surfing and found a ham who uses a wilson fgt2 and fgt4 cb antenna on the ham bands.  The wesbite was hamunverse.  He uses different size tips depending on the band (20-10m)  I am wondering how effective this would be?  Could a hamstick be used the same way?  Has anybody tried this?  Any suggestions?

Randy ka4nma

Posts: 1003

« Reply #1 on: February 25, 2013, 01:55:20 AM »

whatever combination of coil and whip that will get the SWR low enough for your rig to operate can be used. It is easier to use an adjustable coil rather than change whips, but wind load can be a factor. A longer antenna will radiate more signal than a shorter one - less loading coil needed - but the height of the overhangs you encounter is your limiting factor there. It becomes a cost/benefit tradeoff - how many bands will you really want to work, what hours day/night will you be mobile, etc.

Posts: 4013


« Reply #2 on: February 25, 2013, 05:11:44 AM »

Any suggestions?

Posts: 1416

« Reply #3 on: February 25, 2013, 09:13:10 AM »

He uses different size tips depending on the band (20-10m) Could a hamstick be used the same way?  Has anybody tried this? 

Hamsticks might be a bit tough.  Something very similar, though, might be a system like mine:  I've got a 54" aluminum mast mounted at bumper level.  There's a quick release at the top, and then I have Hustler resonators pretuned for each band I want to work.  Say I'm on 20m, to change to 15m I just pop off the 20m resonator/whip and pop on the 15m one.  Not as convenient as a screwdriver, but it works pretty well, and it's cheap, and easy to experiment with.  I use a 12m resonator on 15m, with a longer whip and a capacity hat, a 17m resonator on 20m, and so on--gives a bit more efficiency.    I'll post a link: the car has changed but the antenna is pretty much the same:

BTW, Cecil's "bugstick" is also a really nice possibility.  I've been meaning to build one of those for a long time, just haven't done it yet.  Wink  73 GL!  --ken

Posts: 16661

« Reply #4 on: February 25, 2013, 11:26:17 AM »

Any of the popular mobile antennas with a coil wound on a fiberglass tube
and an adjustable tip section can be modified to a different frequency by
changing the tip length.  It is simply a matter of determining the proper
whip length for each band.

Note that you can't raise the frequency above the self-resonant point
of the antenna without the whip:  while the original author was able to
get a low SWR on 6m, that doesn't mean that the antenna will be
particularly efficient in that mode, depending on the details of the
loading coil.  But starting with an antenna designed for CB with a very
short top whip and replacing it with a longer one will allow operation
on several bands - as long as the antenna will withstand the
unexpected stresses from the wind drag on the extended whip.

Using a Hamstick with a longer whip on the next lower band will
usually be a bit more efficient than a Hamstick designed for that
band because of the longer antenna length.  (The same has been
done using the Hustler mobile resonators.)

It's all a matter of the right combination of loading inductance,
whip length, and how the inductance is distributed along the antenna.
There's no magic here.

But, for the same total antenna length, Cecil's "bugstick" approach
is probably going to be more efficient because it uses a larger
diameter coil.  The long, skinny coils made by winding the wire onto
a fiberglass rod has higher losses.

Posts: 14243

« Reply #5 on: February 25, 2013, 12:05:58 PM »

You can change the resonant frequency on a Hamstick "a little bit" by adjusting the whip but you probably can't make a 75M Hamstick work on 20M, 15M, or 10M because the loading coil wound on the fiberglass section has way too many turns. If you tried to make a 15M hamstick resonate on 75M you would need a very long whip, probably on the order of 20 feet or more.


Posts: 65

« Reply #6 on: February 27, 2013, 01:39:36 AM »

The hamstick makes a great blank for antenna construction, I can't link pictures from my work pc, but if you check out my QRZ page half way down you'll see a picture of a rather large copper coil (5" dia) on a much modified hamstick, this is a multiband antenna that works from 10 - 20m by changingf the position of a jumper wire.
When I made the antenna it was a rough proof of concept exercise, and becuase I use copper car brake tubing the coil is self supporting. I used an antenna analyser so that the shorting position on the coil doesn't require any whip length adjustment, however on 20 and 17m the antenna requires shunt feeding due to high VSWR at resonance, however from 15 to 10m I can get away without shunt feeding.

The antenna is very efficient, if a little large, and has been the main reason for me being able to work 200 DXCC mobile. The hamstick was secondhand and the copper tubing cheap, so for the cost of $50 I have an antenna that would easily compete with much more expensive screwdriver antennas, the downside being manual band change.

Happy mobiling  Wink

Posts: 65

« Reply #7 on: February 27, 2013, 01:58:01 PM »

Now at home I can link to an image of the coil on my big mobile antenna, 8' tall and a 5" centre loading coil, you should also be able to see the various tapping points soldered onto the coil.
A coat of lacquer should stop the coil turning green Wink

Gin still Big Coil

Posts: 28

« Reply #8 on: March 03, 2013, 12:58:22 AM »

WB6BYU is exactly right. Low VSWR doesn't mean an antenna will be effecient, after all, dummy loads are 50 ohms, right?

Posts: 4013


« Reply #9 on: March 03, 2013, 06:55:51 AM »

Low VSWR doesn't mean an antenna will be effecient, ...

The ARRL Antenna Book says the radiation resistance of an 8' center-loaded 75m mobile antenna is about 0.8 ohms. Any feedpoint resistance over that value is due to losses. Maximum radiation of RF from that antenna would occur if the antenna were lossless at which point the SWR at the feedpoint would be 62.5:1. It follows that the most efficient well-designed 75m mobile antennas are the ones with the highest SWR at the feedpoint. My best 75m mobile antenna had a 12.5 ohm feedpoint resistance with an SWR of 4:1. That's when we need a shunt capacitor or shunt inductor at the feedpoint - or a 1:4 unun would have worked.

For 75m mobile antennas in particular, the closer the feedpoint resistance is raised to 50 ohms (SWR=1:1) the lower the radiation efficiency of the antenna. For a 75m mobile antenna, a 50 ohm feedpoint resistance (SWR=1:1) equates to an antenna radiation efficiency of about 1.6%, i.e. 100 watts in yields 1.6 watts out.

Posts: 477

« Reply #10 on: March 05, 2013, 09:10:28 PM »

Can it be done?  Basically you would be using the same loading coil with various sized whips.  So yes, it can be done.  I haven't done it but I'll bet someone has before.  I think there are 'better' ways of doing it, but that certainly doesn't mean those ways would be better for everyone.
 - Paul
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