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Author Topic: Hamstick/Zeppelin dipole?  (Read 2654 times)
KB3RPM
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Posts: 6




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« on: November 08, 2010, 06:39:57 AM »

I live in an area with VERY restricted space. Inner city, power lines, etc. Didn't want to go with the MFJ apartment antenna. so I went with a Zeppelin antenna. 75' with a 50' ground running parallel. End-fed to a simple PL259. Have it only 12-13' above the ground and running against the side of the building. Best I could do. . . .

I'm working 80meters - 20meters, and using a CB antenna for 10meters (all with a tuner of course)  What I have works, but is FAR from ideal. Too many times, I get bad signal reports from stations a few states away, and it's usually attributed to my low antenna height. My idea, is to buy a 75m hamstick. I have one of those hamstick dipole mounts. My thought is to use the zeppelin as the ground side (75') and the hamstick as the radiator. It'll be close to the front of the building, but a good 28-30' in the air.  I guess my long-winded questions are;

1. Are hamsticks multi-band capable? With a tuner, can I continue to cover all bands?
2. This works in theory, but has anyone tried it?
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N3OX
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Posts: 8915


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« Reply #1 on: November 08, 2010, 07:37:33 AM »

1. Are hamsticks multi-band capable? With a tuner, can I continue to cover all bands?

No, unfortunately.  

Quote
2. This works in theory, but has anyone tried it?

A lot of people have tried 80m hamsticks.  They're about 1%-2% efficient at best, making you a QRPer even if you use 100W.  

Your antenna right now is probably  pretty bad, but more like 5%-10% efficient, not 1% efficient.  Plus, a vertical like a hamstick is not good for the kind of local communications you're trying to do.  A horizontal, even a low one, is almost certainly better.

If you have 75 feet you would probably be better off if you want to talk locally on 75m with a shortened dipole antenna installed 12-13 feet up:

http://yb3td.com/?p=40

I think that should be substantially better in about the same space as what you've got now if you can do that.  

Your low antenna height doesn't help, but that's not all of it.  The close spacing between the wire and the ground that's carrying an equal and opposite current causes some field cancellation and a low antenna impedance, and running a ground that's just one wire makes for a pretty lossy ground system.  Just stacks up against you.   Probably being close to the building doesn't help either but sounds like there's not much to do about that.

It's probably better to try for a ground-independent antenna like a dipole if you can.

Can you put stuff on your roof at all?  I suppose not...
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
Member

Posts: 18078




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« Reply #2 on: November 08, 2010, 09:48:40 AM »

Quote from: KB3RPM
... I went with a Zeppelin antenna. 75' with a 50' ground running parallel. End-fed to a simple PL259. Have it only 12-13' above the ground...

I'm not sure why you're calling that a "Zeppelin" antenna, as the standard Zepp is a half wavelength long (or
multiple thereof) and fed with open wire line.

It sounds like you are using a single 75' wire that runs up from the feedpoint, then horizontally, and a single
50' ground radial underneath it that is connected to the coax shield at the feedpoint.  Is this correct?

How long is your feedline?  What type of coax are you using?


Quote
...Too many times, I get bad signal reports from stations a few states away, and it's usually attributed to my low antenna height...


Are those "few states" to the Northeast or to the West of you?  That makes a big difference in the
distance involved.  (I've had thousand mile contacts in the same state before.)

The strength of your signal at a distance depends on the propagation characteristics at the moment and
the required angle of radiation (which depends on the height of the ionosphere, but we can make some
general assumptions.)  Coverage within a couple hundred miles will generally require a high angle of
radiation, which your low antenna can provide, but the bands must be open to support it.  Longer
distances require lower angles of radiation close to the horizon, which will be weaker on a low horizontal
wire (but may be better with a vertical.)


So before we look at your suggested improvement, let's consider reasons why your signal is weak at, say,
250 miles away.

1. Antenna not suitable for the path.  On 40 and 20m your antenna pattern will have nulls in it, where your
signal can be down 20dB or more.  And remember we are dealing in 3 dimensions here - the vertical angles
correspond to various distances from the antenna.  On 20m, for example, your antenna will tend to have
nulls perpendicular to the wire (depending on the exact configuration.)  Yes, height is an issue as far as
the vertical angle of radiation, but you should have adequate radiation even at that height, though getting
it up higher will improve matters.

2. High losses.  I can see thtee primary sources of loss:  ground losses on 40m and especially 80m due to
a horizontal antenna close to the ground, ground return losses due to feeding the antenna against an
inadequate radial system, and coax losses due to high SWR.  The first depends on ground conditions,
and there may not be much you can do about it.  The second is improved by adding more ground radials,
or by using an antenna such as a dipole that is not fed against ground.  The last one is a function of the
SWR at the antenna (not what the rig sees after the tuner) and the length and type of coax you are using.
But a 75' wire without any matching is going to have a high SWR on 80m and 40m, though you might get
lucky on 20m.  Improving the impedance match at the antenna end of the coax may make a big improvement,
and there may be other issues with your current installation as well.

3. Poor propagation conditions.  Especially with a marginal antenna you have to choose a band that is open
for the desired distance and direction.  With the current conditions here in Oregon, 40m is only open for
distances OVER 200 miles, and only in the middle of the day.  80m won't support local communications during
the night (though it does during the day.)  Your conditions may be different, and it varies from day to
day.  The Australian Ionospheric Propagation Service has some handy tools to see what bands can be
expected to be open:

http://www.ips.gov.au/HF_Systems/7/1

I use the Local Area Mobile Prediction (LAMP) charts for a quick check of what bands are most suitable
for different distances throughout the day, but the Hourly Area Predictions (HAP) may be useful, too.


Quote
My idea, is to buy a 75m hamstick. I have one of those hamstick dipole mounts. My thought is to use the zeppelin as the ground side (75') and the hamstick as the radiator. It'll be close to the front of the building, but a good 28-30' in the air.  I guess my long-winded questions are;

1. Are hamsticks multi-band capable? With a tuner, can I continue to cover all bands?
2. This works in theory, but has anyone tried it?

1. No.  The coil is very close to self-resonance in order to get enough inductance to load the antenna on
80m, and operating it above that frequency will tend to be very inefficient even if your tuner matches it.

2.  No, it doesn't even work in theory.  To work 80m you'd want a radial that is quarter wave resonant,
or about 60 to 65 feet.  Otherwise you can't tune the hamstick to resonance.  And you'll likely still have
a lot of current flowing on the coax with only one radial.  But it could work that way for a narrow segment
of 80m - the wire would radiate much more than the hamstick, and if you could get it up to 28' that would
be an improvement as well.

Just because your tuner can match the impedance doesn't mean it will work as an antenna.

Without knowing more about your specific situation, I'd suggest a half-wave dipole for 40m (about
60+ feet long) would fit the space and give a good match to your coax.  Then you can add a set
of 20m elements to it.  See if that works any better than your existing antenna.  Adding 80m takes
a bit of work (since a full dipole would be 120' long) but could be done using loading coils on the ends
of the 20m elements.

If there is a specific location you want to work, we can evaluate the performance in that direction at
the required vertical angle.
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