40/15M dipole question
cark...@juno.com
7.200 mhz. I'll probably use 14-16 gauge insulated house wiring for
this antenna. It'd be a "nominal' quarter wavelength per leg.
This dipole will be fed with 50 ohm coax, and it will be erected
horizontally, 30 feet high over "average" ground.
What SWR reading could I expect to see from this antenna if I tried
to use it at 21.300 mhz........in the same environment as stated
above?
Thanks, folks......
FjLee KA0FPJ
Cecil Moore
> What SWR reading could I expect to see from this antenna if I tried
> to use it at 21.300 mhz........in the same environment as stated
> above?
EZNEC sez 11:1 with a feedpoint impedance of 100-j200 ohms.
--
73, Cecil, w5dxp.com
Richard Clark
<cark...@juno.com> wrote:
>What SWR reading could I expect to see...?
15:1
73's
Richard Clark, KB7QHC
No Spam
Google a fan dipole. It sounds like something you may be interested in
John - KD5YI
Hey Cecil -
I got:
Impedance = 85.26 - J 186.3 ohms
SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308
Let's see if we can determine why they don't match.
I adjusted the wire length for no reactance at 7.2 MHz.
I used 99 segments of #14 wire. I did not include insulation effects.
How about yours?
John
John - KD5YI
Oops! I just noticed that I had zero wire resistance. Changed it to
copper but only got a slight change:
Impedance = 87.47 - J 184 ohms
SWR (50 ohm system) = 9.960 (75 ohm system) = 7.042
nm...@wt.net
The losses won't be enough to really worry about.
Jim Lux
If you tune it down a bit so you resonate at perhaps 7.15, then the
third harmonic will be closer to 21.3. It's a dipole, so not real high
Q, particularly on the fundamental, so maybe what you do is go for a
compromise tuning.. A bit low on 40 and a bit high on 15.
John - KD5YI
Well, yeah, I know. But, I'm trying to simulate based on the OP's info
and Cecil's results.
John
John - KD5YI
Are you using 9 segments?
John
tom
I think may have missed something on how segmentation works. I thought
you needed an even number of segments to split a dipole evenly.
So while 99 is close to centered by my logic, if 9 was used it might
skew things a bit.
If I'm off base please tell me so I can adjust my model of how NEC works.
Thanks.
tom
K0TAR
John - KD5YI
Hi, Tom -
As far as I am aware, using an odd number of segments in a wire allows
the source to be placed in the center of a segment with equal numbers of
segments on each side.
Think of it this way: a source needs to be in the middle of a segment.
So, you need an even number of segments on each side to keep everything
balanced. If so, then the total number of segments needs to be odd.
If this does not make sense, please ask again and I'll try with more detail.
Cheers,
John
Kickin' Ass and Takin' Names
Just put it up and see what happens.
Slap a tuner in the line.
Or check out a fan dipole.
Or buy a pair of 15 meter traps and use the design sheet that comes
with them to build a 40-15 trap dipole.
http://www.unadilla.com/?p=34
Cecil Moore
> How about yours?
I used an EZ file that I already had. It was 40 feet high and I
rounded off the feedpoint impedance. I thought that would be a close
enough approximation.
--
73, Cecil, w5dxp.com
nm...@wt.net
But I've never bothered with legs for 15m..
Always used the 40 m legs for 15m.. Guess a habit back
from my novice days.. Back then, my old transmitters
would match either load no sweat.. No tuner required..
Heck, I didn't even use an SWR meter when I first
started.. :/ Never missed it either..
With my modern rigs, the match on 15m, using the
40m legs is usually a tad higher than I'd like for
S/S finals.. :(
So I slap a tuner into the picture as a "line
flattener". Even a cheapy MFJ random wire tuner will
match that.. And most internal radio tuners could
probably handle it ok.
It's not too far off from being a 3/4 wave on 15m.
None of my radios have internal tuners, but
I have several stand alone tuners.
cark...@juno.com
........and OUCH!!!.......
The swr is higher than I had hoped for. I was hoping to use a duo-
band 15/40 meter wire dipole that could be fed directly with 50 ohm
coax connected to my Yaesu FT-450AT with internal tuner.
But the FT 450 AT internal tuner will only match up to a 3:1 swr, so
that won't be "do-able".
Thank you again......
FjLee KA0FPJ
Denver CO
tom
>
> Hi, Tom -
>
> As far as I am aware, using an odd number of segments in a wire allows
> the source to be placed in the center of a segment with equal numbers of
> segments on each side.
>
> Think of it this way: a source needs to be in the middle of a segment.
> So, you need an even number of segments on each side to keep everything
> balanced. If so, then the total number of segments needs to be odd.
>
> If this does not make sense, please ask again and I'll try with more
> detail.
>
> Cheers,
> John
Thanks! I guess I'll have to look into the docs a bit to educate myself.
tom
K0TAR
Gary
See QST Dec. 1991 Hints and Kinks. The author cut the dipole for the
low end of 40M~67' and used a Q section of RG59 cut for 21 MHZ to
improve the match on 15M.
Gary N4AST
John - KD5YI
I understand. Of course, it's close enough. I was just wondering why my
answers did not agree with yours, but now I do.
Thanks,
John
Jim Lux
I always use an odd number of segments, and the middle segment is where
I feed it. The voltage source is essentially in series with the
segment, and if your segments are short enough, the current distribution
along the segment is effectively uniform, so it makes no difference.
OTOH, I'm running NEC4 with a lot of memory in double precision, so
using hundreds of segments isn't a big deal.
Jim Lux
I think it *will* work (based on practical experience.. I've done this a
bunch of times).. you just need to fiddle with the length to get an
acceptable match in both bands. And, of course, you're not going to get
something which matches the bottom of one band and the top of the other.
People fling up dipoles (theoretical feedpoint Z around 70 ohms) and
feed them with 50 ohm coax all the time and never give it a second
thought. (partly that's because rarely is the dipole perfectly flat..
if it droops or peaks up, the Z gets closer to 50 ohms)
Dave Platt
>> band 15/40 meter wire dipole that could be fed directly with 50 ohm
>> coax connected to my Yaesu FT-450AT with internal tuner.
>>
>> But the FT 450 AT internal tuner will only match up to a 3:1 swr, so
>> that won't be "do-able".
>I think it *will* work (based on practical experience.. I've done this a
>bunch of times).. you just need to fiddle with the length to get an
>acceptable match in both bands. And, of course, you're not going to get
>something which matches the bottom of one band and the top of the other.
I agree, it may very well work acceptably. I've been able to get a
similar 40-meter wire to match up OK on 15 meters, using the autotuner
built into my Kenwood TS-2000.
Another option would be to add some sort of matching element, either
at the antenna or at the rig, which would bring the feedpoint
impedances on both bands to within the ranges acceptable to the rig's
internal tuner.
If the antenna is close to 50 ohms resistive in the 40-meter band, and
is around 85.26 - J 186.3 in the 15-meter band, then you could
consider adding a series coil at the antenna feedpoint. If I'm doing
the numbers right, an inductance of 1.5 mH at the feedpoint would
cancel out the 186 ohm capacitive reactance almost exactly, leaving
you with an 85-ohm resistive impedance or an SWR of around 1.7:1, which
should be quite easy to drive.
This much inductance would change the feedpoint Z on 40 meters to
around 50 + J60, so the SWR would be quite a bit more than 1:1...
probably still within the range of your tuner, though.
Several ways to deal with this:
- Compromise, by using less series inductance... maybe 1 uH - this
wouldn't lower the SWR on 15 meters as much, but would raise the
SWR on 40 meters less. Calculate and fiddle a bit and you can
probably find an inductance which will result in similar (and very
comfortable) tuning SWRs on both bands.
- Instead of a plain L at the feedpoint, use a series LC - one which
is series-resonant at 40 meters (thus adding no reactance and not
changing the SWR at all) and has a net reactance of J186 at 15
meters.
All of these techniques will narrow the SWR bandwidth on 40 meters
somewhat, but with care you should end up with an easily-tunable
antenna.
--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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Dave Platt
In article <bgi178-...@radagast.org>,
Dave Platt <dpl...@radagast.org> wrote:
>If the antenna is close to 50 ohms resistive in the 40-meter band, and
>is around 85.26 - J 186.3 in the 15-meter band, then you could
>consider adding a series coil at the antenna feedpoint. If I'm doing
>the numbers right, an inductance of 1.5 mH at the feedpoint would
>cancel out the 186 ohm capacitive reactance almost exactly, leaving
>you with an 85-ohm resistive impedance or an SWR of around 1.7:1, which
>should be quite easy to drive.
... for which, please read "1.5 uH" - micro rather than milli :-(
Similar errors elsewhere in my posting.
Alejandro Lieber
A very simple solution in:
http://www.astrosurf.com/luxorion/qsl-g5rv-2.htm
Near the end, look for:
How to tune a 40-meter dipole antenna on 15 meters ?
Alejandro Lieber LU1FCR
Rosario Argentina
Real-Time F2-Layer Critical Frequency Map foF2:
http://1fcr.com.ar
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