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1-10 of 12 messages
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Increasing Antenna Bandwidth??
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by KB9WIS on October 11, 2009
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Anyone have some ideas on how to improve the bandwidth of a vertical antenna? It's a 5/8th wave 40' vertical design, and I'm using a porbable fiberglass mast, so can't really enlarge the antenna conductor size more than 10 gauge wire. The bottom of the antenna can support a bigger coil however..
For tuning, I'm only using an inductor at the feedpoint/bottom.. I'm assuming since I can't add anymore antenna conductor size/weight to this mast above it, my only option is to make the inductor less efficient and reduce it's Q? What would be the best way to reduce a coils Q? Should I take out the copper tubing and use a tin plated and/or a stainless steel wire (I'm assuming the added losses of these conductors would reduce the Q). May sound weird wanting to make a lower Q inductor/coil, but I think this is my only option? In this case, I would like to improve my bandwidth 30-50%, even if there is a few db of loss..
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RE: Increasing Antenna Bandwidth??
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by WB2WIK on October 11, 2009
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Increase bandwidth for what band?
What is the actual bandwidth you're trying to achieve? (Not "30% more," but what actually, from x MHz to y MHz?)
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RE: Increasing Antenna Bandwidth??
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by WB6BYU on October 11, 2009
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While 40' is nominally 5/8 wave on 20m, I had to extend
it to 47' (using #18 wire and Mininec ground in EZNEC)
to get a 50 ohm input impedance with a 3uH series coil.
At that point the SWR bandwidth was about 300kHz at 2 : 1.
By shortening the wire to 41' the resistive component was
200 ohms, and using a 9uH coil with a 4 : 1 un-un gave a
bandwidth of 600kHz (13.9 to 14.5 MHz.)
Are you getting a low SWR using the current method?
A 40' wire should give a low SWR on 17m without any coil,
and the SWR bandwidth should easily cover the band. But
the antenna is operating as 3/4 wave rather than 5/8,
with the associated high angle lobes.
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RE: Increasing Antenna Bandwidth??
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by KB9WIS on October 11, 2009
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WB6BYU, good tip about creating a purely resistance load and using a step down transformer/4:1 balun to bring it down to 50 ohms from there. I'm sure the reactance is a large culprit, as the inductor has to both tune out by the capacitive reactance and bring impedance down to 50 ohms, the point where both are ideally aligned makes for more narrow bandspread.
The only problem, is that I've had a hard time finding a 4:1 balun that actually handles legal limit RTTY/Carrier. If you are confident of any, let me know.
I'm using this for 17-20 meters, 5/8th wave design and it also works 40-75 meters as well.. Since I have a long coax run (400'), I'm trying to keep all SWR. I may think about the balun approach, if I can find a great 4:1 balun avail commerically.
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RE: Increasing Antenna Bandwidth??
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by WB6BYU on October 11, 2009
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First, you do NOT want a balun. You want an Unun, with
a common ground connection for the antenna and the coax.
There was a long thread previously on this topic, and it
seems that a couple of manufacturers who used to provide
4 : 1 baluns with their 41' verticals have switched to
heavy-duty Ununs instead. Any of those should be
suitable in this application.
Both the resistance and reactance change with frequency.
With my model (which isn't perfect, but adequate for
this discussion) the antenna is resonant at 53 ohms on
17m and the impedance is 220-j800 ohms on 20m. The
series coil does NOT lower the feedpoint impedance -
that requires a shunt coil. It only cancels the reactance.
Clearly, with a 4 to 1 change in resistance between the
two bands the best you could do would be to design a
100 ohm feed and accept a 2 : 1 SWR at best, even if
you switched in the series coil on 20m. Probably a
better approach would be to use a switch or relay to
connect the coax directly to the antenna on 17m and
to the input of the 4 : 1 Unun + series inductor on
20m.
There may be a clever circuit that you can use to match
it on both bands, like the common for 40/20m (where it
is a quarter wave on one band and a half wave on the
other.) One way to start is to design an L network to
match the impedance on 20m, then replace each leg with
a series or parallel combination that gives the desired
reactance on 20m while appearing as a short/open (as
needed) on 17m. With the frequencies so close together,
however, this likely will narrow the bandwidth on each
band.
By lengthening the antenna a bit (around 44') you can
make it so the resistance is around 90 ohms on both
bands, which would allow you to just switch the series
coil (for 20m) and capacitor (for 17m). But the
resistance changes fairly rapidly with length in that
range, and the wire would have to be trimmed within a
fraction of a foot.
When in doubt, use a double pole relay at the feedpoint
for both the antenna and the coax. That way you can
build whatever matching network you need for each band.
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RE: Increasing Antenna Bandwidth??
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by WB6BYU on October 12, 2009
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Minor correction: a shunt coil is used to RAISE the
feedpoint impedance, not to lower it.
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RE: Increasing Antenna Bandwidth??
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by N5LRZ on October 12, 2009
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Bandwidth increases as the dimensions of the radiator increases.
Example: a 3/4 inch copper pipe will have a greater bandwidth compared to a 1/4 inch copper pipe.
Increase the physical size of your radiator.
Google Cage Dipole.
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RE: Increasing Antenna Bandwidth??
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by KB9WIS on October 12, 2009
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I can't increase the diameter anymore.. I'm already using 3 x 12 gauge wires spaced, and it's already too heavy.
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RE: Increasing Antenna Bandwidth??
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by N5LRZ on October 13, 2009
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RE WIS...
Then pick the segment you can work and screw the rest.
SOMETIMES we don't get what we want in this world.
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RE: Increasing Antenna Bandwidth??
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by WB6BYU on October 13, 2009
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Actually, making the antenna a larger diameter might be a
good idea, and not as bad as it sounds. Replace the big
wire you have on there now with something thinner - like
#18 or #24 stranded, insulated hookup wire. Arrange for
some sort of spreader near the middle of the pole: try for
4' or so (2 feet on each side of the mast). This might
be either a lightweight piece of fiberglass tubing or
strings tied to the wires to pull them sideways.
Run one wire straight up the pole, and one to each end of
the spreader and back to the top, where they are all connected
together. That gives you a 4' wide vertical (at least in
the middle).
According to my EZNEC model (using MiniNEC ground, so not
perfectly accurate) that lowers the input impedance of the
antenna when it is half wave resonant (near 20m) while
still providing 3/4 wave resonance around 17m (the
exact resonant frequency will change.) But if you are
going to use a tuner in the shack you don't have to
worry about that: put the big 4 : 1 UN-UN at the base
and the SWR shouldn't be much worse than 4 : 1 from
9 to 30 MHz. (If you can make it wider you can cover
40m as well with the same SWR range.) But even if the
SWR gets up to 5 : 1, that is low enough that the
losses aren't too bad using coax feed to the shack.
Note that for this to work the radiator has to be fat
in the right places to lower the impedance when it is
a multiple of half wavelength. Once you do that the
SWR is highest at 18 MHz where the antenna is 50 ohms,
which gives a 4 : 1 SWR using the 200 ohm UN-UN. If you
use a single wire the input impedance is too high at
the half wave resonance.
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