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Author Topic: Webster Band Spanner antenna  (Read 2715 times)
KJ4RQV
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« on: September 08, 2011, 09:09:17 AM »

I was given a Webster Band Spanner mobile antenna and want to try using it as a fixed mono band unit. I'd like to use a tilt over pole to put it about ten feet up but I am wondering if I should use radials and if so, how or where to attach them--can they just be afixed to the bottom, non-tilting section?
I just like fooling with antennas and mostly am working QRP so it's all for fun.
Don
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KJ4RQV
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« Reply #1 on: September 08, 2011, 09:13:56 AM »

I may have found my answer in the post About Locating a vertical but am open to comments.
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N3OX
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« Reply #2 on: September 08, 2011, 09:20:24 AM »

What band do you want to run?

If it's a lower band, like 40m, you'd probably get the best mileage from it if you used it as an adjustable top load for a 10 foot metal pole and put radials on the ground or elevated resonant ones attached to your tilt base.  In that case you would not connect the coax to the band spanner at all, but just connect the top of the pole to the bottom of the bandspanner's coil and feed the pole against the radials.  You would still pull the whip in and out for adjustment of the resonant frequency.

I suspect that it would work better on most bands this way, because the efficiency and wider bandwidth of a tall vertical lower to the ground would be more advantageous than having a short vertical elevated only 10 feet.

You can also treat the mast as just a mast but then you want to run the radials from the top of the mast.  In any case the shield of the coax should attach directly to the radial common point, wherever that is.  I wouldn't run coax up the mast and put radials at the bottom.  That will probably "work" in a fashion but you'll have problems with currents on the coax shield and you will get a lot of weird uncontrolled behavior that way.
« Last Edit: September 08, 2011, 09:23:00 AM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KJ4RQV
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Posts: 130




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« Reply #3 on: September 08, 2011, 10:04:46 AM »

Thanks, that is the info I was looking for. I will most likely be on 40 or 20 meters.
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WB6BYU
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« Reply #4 on: September 08, 2011, 12:08:17 PM »

Quote from: KJ4RQV
I will most likely be on 40 or 20 meters.

That makes a big difference in this case.

On 40m using the mast as part of the radiator improves efficiency.  On 20m adding 10' to the
portion below the loading coil may make the antenna too long to tune properly.  I don't
remember what the minimum length is where the slider makes contact with the coil, but I
think it is close to 8' long (so 10m uses little or no inductance.)  The combination would
have a minimum length of 18', which is longer than a quarter wave on 20m.  But an 8' mast
should work.

And if your objective is to experiment with antennas, this might be one aspect to look at.

But the radials should attach where the coax shield does, not to the bottom of the mast.
Otherwise the shield ends up having some RF voltage above ground, which aggravates
common mode currents.
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N3OX
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« Reply #5 on: September 08, 2011, 12:58:51 PM »

On 20m adding 10' to the
portion below the loading coil may make the antenna too long to tune properly. 

 Oh I was looking at the "Short" bandspanner which has a fully extended length of 93 inches, so there should be some intermediate setting that resonates on 20m with a 10 foot mast.  But the regular one when fully collapsed seems to go down to 63 inches.

http://bama.edebris.com/manuals/webster/bandspanner
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #6 on: September 08, 2011, 04:07:34 PM »

The key piece of data is that the adjustment range of the whip is 24" in both cases.  When the antenna
is shortened more than that much from the maximum height, the internal slider is no longer in contact
with the coil.  So it can be telescoped in to save space, but has to be within 24" of maximum length
in actual operation.

So the short version will be a minimum of 69", and the long version will be at least 93".  With an
added 10' mast, the minimum length is either 15 3/4 feet or 17 3/4 feet.  Since a quarter wave
on 20m is around 16 3/4 feet (depending on the diameter, etc.) the shorter one should work, but
the longer one probably will need either a shorter mast or a series capacitor at the feedpoint to
bring it to resonance on 20m.  (The matching point will be close to minimum inductance in either
case - that is where you feel the resistance as you pull the whip out and the slider starts to
contact the coil.  Don't expect it to match at the normal 20m marking.)

Of course, you could also try it with an elevated feedpoint as originally proposed, in which case the
length of the mast doesn't matter (as long as the radials are connected at the feedpoint.)


Now you've got me thinking about digging my own antenna out of the barn and doing something with
it.  I used to have a borrowed BandSpanner, but some years ago picked up a later AllSpanner(?) instead.
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KJ4RQV
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Posts: 130




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« Reply #7 on: September 08, 2011, 04:13:53 PM »

Great info!
I am learning so much here. I wish I didn't have to work tomorrow as I want to get to play with this.
This is one of the most fun parts of the hobby to me.
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