Stub Tuned Dipole - Will it work??
andrew...@hp.com
tried it and if it would work?
My idea is to take an 80M dipole and place tuning stubs (short 1' pieces of
wire) at resonant points for the other bands to achieve multiple resonant
points.
Like so:
0 = insulators
x = connection at resonant points, minus length of stub
<-10M dipole->
<--15M dipole-->
<--------20M dipole------>
<---------------40M dipole------------->
<------------------------80M dipole-------------------->
0---------x-----x----x---x---0---x---x----x------x---------0
\ | | | | /\ | | | | /
\ | | | | / \ | | | | /
\0_____0____0___0/ \_0___0____0______0/
^
1' long stubs hang from dipole
and are insulated from rope at
bottom of stub.
Resonance for each band is
adjusted by sliding stub left
or right until resonance is
achieved.
Has anyone tried this approach before? What were the results? Did the
antenna work?
Thx de Andy, KD3RF
-----------== Posted via Deja News, The Discussion Network ==----------
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W6RCecilA
> My idea is to take an 80M dipole and place tuning stubs (short 1' pieces of
> wire) at resonant points for the other bands to achieve multiple resonant
> points.
Is the theory to build a trap dipole using stubs for traps?
--
73, Cecil, W6RCA http://people.delphi.com/CecilMoore
Goldreged
and are insulated from rope at
bottom of stub.
Resonance for each band is
adjusted by sliding stub left
or right until resonance is
achieved.
Has anyone tried this approach before? What were the results? Did the
antenna work?
Thx de Andy, KD3RF>>
Almost seems SO Simple that it must have been tried. I havent heard about it,
though...
Ed G.
(Please Remove "JUNKBLOK" to E-mail.
Unless it's a Flame.)
Check This Site !!!
http://www.charityweb.net/Safehaven
Alan Taylor
Interesting stuff! I have done some work on this but it takes a lot of ' cut
and try '. The simplest was a top loaded 14Mhz mobile whip with a short
about 1ft) stub beneath it (sticking out slightly above horizontal). By
adjustment this enabled the antenna to resonate on both 21Mhz and 14Mhz. It
saved stopping getting out of the car when changing bands! Another idea for
the 80m dipole is to thread groups of ferrite beads onto the wire alongside
the stubs. I'd be interested to hear how you get on. Sadly my antenna
pruning hours are greatly reduced by having to go out to earn the dollar!
73 de Alan VK6BN
andrew...@hp.com
I don't know if I would consider the stub to be a trap, although in reality
that's what it may look like electrically.
The way I was thinking though is that on a random dipole resonant points for
ham frequencies occur at points all along the line. By adding the 1' stubs
and "sliding" them along that line, at some point, the stub + the active
portion of the dipole + the "unused" portion of the diploe must resonate on
the desired frequency.
Essentially what I am doing is taking a fan dipole and "bundling all of the
fanned wires together, except for the final 1' of each wire, which is then
allowed to hand straight down.
I haven't tried it, but am thinking of putting one up.
73 de Andy, KD3RF
In article <3640BA...@ibm.net>,
-----------== Posted via Deja News, The Discussion Network ==----------
andrew...@hp.com
Thx for the reply... Yes, I agree that a lot of cut & try will be required. I
use the Autek HF Analyst and with it and some basic math I think I can get in
the ballpark fairly easily.
In fact, I was thinking last night that I could possibly broadband the stub
tuned dipole on 80M by hanging multiple stubs for the band, separated by just
enough distance to give more or less continuous coverage on the frequency
range of choice.
I do expect, though, to see antenna feedpoint resistance/impedance dance all
over the place even though resonance can most likely be achieved.
Will most likely try winding baluns to solve that issue if I can get
impedance(s) of each band to come out near the same. See Jerry Sevik's book
on baluns (ARRL) for some interesting reading.
Does this make sense?
de KD3RF, Andy in Kennett Square, PA
In article <iv402.20$s86.1...@nsw.nnrp.telstra.net>,
"Alan Taylor" <ata...@tnet.com.au> wrote:
> Andy,
> Interesting stuff! I have done some work on this but it takes a lot of ' cut
> and try '. The simplest was a top loaded 14Mhz mobile whip with a short
> about 1ft) stub beneath it (sticking out slightly above horizontal). By
> adjustment this enabled the antenna to resonate on both 21Mhz and 14Mhz. It
> saved stopping getting out of the car when changing bands! Another idea for
> the 80m dipole is to thread groups of ferrite beads onto the wire alongside
> the stubs. I'd be interested to hear how you get on. Sadly my antenna
> pruning hours are greatly reduced by having to go out to earn the dollar!
>
> 73 de Alan VK6BN
>
> >
> >
James Aeschliman
I haven't tried this, but saw a discussion of this on FIDONET. One of
the posters stated it would work.
In a previous article, andrew...@hp.com () says:
>I have an idea for a variation on the fan dipole and I wonder if anyone has
>tried it and if it would work?
>
>My idea is to take an 80M dipole and place tuning stubs (short 1' pieces of
>wire) at resonant points for the other bands to achieve multiple resonant
>points.
>
>Like so:
>
> 0 = insulators
> x = connection at resonant points, minus length of stub
>
> <-10M dipole->
> <--15M dipole-->
> <--------20M dipole------>
> <---------------40M dipole------------->
> <------------------------80M dipole-------------------->
>
> 0---------x-----x----x---x---0---x---x----x------x---------0
> \ | | | | /\ | | | | /
> \ | | | | / \ | | | | /
> \0_____0____0___0/ \_0___0____0______0/
>
> ^
> 1' long stubs hang from dipole
> and are insulated from rope at
> bottom of stub.
>
> Resonance for each band is
> adjusted by sliding stub left
> or right until resonance is
> achieved.
>
>
>Has anyone tried this approach before? What were the results? Did the
>antenna work?
>
>Thx de Andy, KD3RF
>
--
A.D. 1999 - The LAST year of the 1900's.
A.D. 2000 - The LAST year of the 20th Century.
Jim Aeschliman bb...@scn.org
Black Diamond, Washington W7WJM
Alan Taylor
Andy,
I am also into baluns and broadband transformers, Jerry's book is my main
reference too!
I thought about using some of that computer ribbon cable as basis for a
multiband dipole. I have some cable that has a heavier than normal conductor
with greater spacing. The conductors would be cut to gradually reduced in
length towards the middle. Problem is the conductors are really too close to
each other but I guess you could arrange for them to individually fan out at
the ends. Other problem would be arc over at the high voltage ends (unless
they are separated) but probably be OK for QRP. What you think?
73 de Alan VK6BN
JDer8745
I think!
Thus the length of each stub must be chosen accordingly.
73 de Jack, k9CUN
Richard Ferch
In reponse to your original posting (quoted below), I agree with Jack,
K9CUN; I don't think it will do what you want.
I'm surprised you haven't had more feedback on this. I guess everybody
on this newsgroup is either embroiled in theological arguments (complete
with quotes from IEEE and Webster as scripture; why don't those guys
just argue about how many angels can dance on the point of a pin, it
would be no less constructive -- give it up, guys, and get back to the
subject of the newsgroup), cross-posted arguments on incentive licensing
(please, leave those to the policy newsgroup where they belong), or
invective against CBers (which doesn't belong here either). Yours at
least was an interesting post about antennas.
My first reaction is based on a gut response: this just sounds too good
to be true, so it probably is. It has a something-for-nothing flavour.
But let's look at it a bit more closely than that.
You have described your idea as a variation on the fan dipole concept.
However, as others have stated in this newsgroup, fan dipoles work best
when the dipoles for the various bands are well-separated. The closer
they are together, the more the resonant frequencies are shifted (this
is especially true for the shorter dipoles). If all of the dipoles in a
fan antenna were separated only by the thickness of the insulation, my
prediction would be that all of the resonant frequencies would be
shifted towards the odd harmonics of the fundamental of the longest
dipole(80m), i.e. you wouldn't see much difference between such a fan
dipole and a simple 80m dipole.
Your proposal is different, however, in that you have eliminated the
insulation, i.e. you have used the same wire for all of the elements
in the fan (except for the last 1 foot of each element). In my view, the
fact that there is no electrical separation between the elements makes
this fundamentally different from a fan dipole (just as serial and
parallel networks are different from one another).
Without the 1 foot "stubs", your suggestion is simply an 80 metre
dipole. Let's look at the "stubs" a bit more closely. What is the
significance of the 1 foot length? It's very small compared to a
wavelength on any of the bands. So, what would happen if it were 1 inch,
or 1 centimetre, instead? Would it make any difference? If not, then I
have a great idea for constructing an all-band antenna -- just make an
80 metre dipole out of barbed wire, and it will be close to resonance
from 80 metres all the way up to 2 metres or so, sort of like a T2FD but
without the losses in the resistor! Ridiculous? I agree. So, the length
of the "stub" must matter. And what is the appropriate length of the
"stub"? I suggest that it would be a quarter wavelength. Make the 40
metre "stub" 10 metres long, attach it 10 metres out less the length of
the "stub", i.e. at the feed point, and what have you got? An
honest-to-gosh fan dipole (assuming you don't just let the 40 metre
"stubs" droop, but pull them out at an angle to the 80m dipole).
You'll notice I have put the word "stub" in quotation marks. This is
because I believe the way you have used it is different from the usual
usage, which is for a length of transmission line. If you were to cut
your 80 metre antenna one foot out from the point where you have a
"stub" for 40 metres, solder the cut ends to the two wires of a piece of
ladder line, and join the wires of the ladder line together at the far
end, you would have something closer to the conventional usage of the
word stub. And if I remember correctly, the appropriate length for such
a stub would be a quarter wavelength. I think an 80 metre antenna with a
pair of 10-metre long transmission-line stubs hanging from it would give
you an
80/40-metre antenna. This would be quite a contraption; it seems an
awful lot simpler just to use a conventional LC trap.
Actually, you could consider a length of wire to be a special sort of
transmission line. But again, the length you would need would be far
more than one foot. A one foot length of wire would act mainly as a
small
capacitance at HF. In fact, I have a multi-band trap vertical in my
backyard with a similar sort of capacitance "hat"; it's in the form
of an X, with the arms about 1 foot long. This capacitance hat does not
act like a trap; about all it does (I think) is allow that segment of
the antenna to be a bit shorter than it would have been without it.
And that leads me to make a bold prediction on how your antenna would
behave if you built one. Let's suppose the 80 metre dipole was cut for
3.6 MHz. After the "stubs" are added, my prediction is that the
resonant frequency will be lowered a bit. For the sake of argument,
let's say the new resonant frequency is 3.55 MHz. What will the
antenna's
feedpoint impedance be on 40, 20, 15 and 10 metres? Calculate the
feddpoint impedance of a simple dipole cut for 3.55 MHz and I'll bet
you won't be far off. Perhaps someone with antenna modelling software
could verify whether this is correct or not. If I'm wrong, I'd sure be
interested to know, because I'd like for your idea to work. I just don't
think it will, though.
My advice to you would be to put up a simple dipole. If possible,
feed it with ladder line and either use a tuner or use Cecil's idea for
varying the length of the feedline. If you can't use ladder line, just
feed it with coax (use a choke balun) through a tuner and live with the
losses in the coax. Or, run a balanced feed line part way, connect it
to a coax feeder through a 1:1 balun, and use a tuner. Cut the antenna
to 102 feet, cut the balanced feed line to 33 feet (or is it 38 feet? I
can't remember off-hand), and you've got a G5RV. It may not be as good
as Cecil's antenna, but it works. I've got one mounted as an inverted
vee above my roof. Over the last 5 years I've managed to work 5BDXCC
with it (plus the ground-mounted trap vertical I mentioned earlier)
through the bottom of the sunspot cycle using a 100-watt transceiver.
This setup can't come close to the typical tribander on a tower, and
it's tough to compete in a big pile-up, but with a bit of patience (and
staying away from the mob that's created every time someone posts a DX
station on the packet cluster -- I wish the packet cluster had never
been invented!), you can work lots of DX, have fun in contests, etc. It
isn't the answer for everybody, but it works for a lot of us.
73 de Rich, VE3IAY
ava...@erols.com
Wow! Thanks for a well thought-out analysis. After reading your reply
several times I think you are right. The barbed-wire analogy really got me
thinking, and your catch on my misuse of the word stub is right-on. I guess
I fell into the "something for nothing" trap. Next project - perpetual
motion or cold fusion - I haven't made up my mind yet, hi!
I also agree with you that the "stubs" as I describe them will most likely
behave as very small capacity hats and not as I originally thought.
I am currently running a G5RV and have also run the tuned dipole with a 4:1
remote balun. Both performed quite well, but the G5RV is better behaved
across a greater span of frequencies than the dipole/remote balun set-up
which is beyond my tuner's capabilities on some frequencies.
In actuality the idea is a father/son science project and I am helping my 14
year old with a physics experiment. So I figured "If I can get a working
antenna out of the project, why not?", and it will be fun anyhow. We are
researching the various popular publications and he is learning the concepts
of resonance and reactance.
We will most likely still construct and test the project, empirically, and
with instruments (I use the Autek HF Analyst, and have tried the various
"NEC" analysis programs, but modeling the collapsed-fan antenna is a bit
tough to express in the software).
I had planned to take measurements with the Analyst at the various frequencies
of interest to see what turned-up in terms of resonance, impedance, and XL and
XC. If you are interested, I'd be glad to share my results with you.
73 de Andy, KD3RF
In article <3647C4...@storm.ca>,
-----------== Posted via Deja News, The Discussion Network ==----------
Reg Edwards
--
******************************
Reg, G4FGQ Click below.
http://www.btinternet.com/~g4fgq.regp
******************************
Gary Coffman
>And that leads me to make a bold prediction on how your antenna would
>behave if you built one. Let's suppose the 80 metre dipole was cut for
>3.6 MHz. After the "stubs" are added, my prediction is that the
>resonant frequency will be lowered a bit. For the sake of argument,
>let's say the new resonant frequency is 3.55 MHz. What will the antenna's
>feedpoint impedance be on 40, 20, 15 and 10 metres? Calculate the
>feddpoint impedance of a simple dipole cut for 3.55 MHz and I'll bet
>you won't be far off. Perhaps someone with antenna modelling software
>could verify whether this is correct or not. If I'm wrong, I'd sure be
>interested to know, because I'd like for your idea to work. I just don't
>think it will, though.
Your analysis is sound. The antenna as described will perform
basically the way you indicate, IE it won't be anything special.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Goldreged
"A 40M And 15 M Dual Band Dipole"
basicly does the same thing. it's a 40 M Dipole w/ "Figure Eights"(2' #14 OR
#12 solid copper) spaced out at the 1/4 wave point for 15 M.
Just fiddle with the 8 to tune for center freq.
artu...@my-dejanews.com
for this father and son project. Secondly I would like to give him some good
reasons to pursue his original thoughts with some comments that are contrary
to the other responses.The idea does has merit! The poster recognises that an
antenna will work as long as you give the electrons a repeatable path at a
particular frequency.As with the log-periodic many paths are provided with
the multiple elements. Only those elements that give repeatable paths as per
above will be active. If the "stubs" are closed at the ends as well as being
part of the primary dipole then there will be current flow around the "'stub"
tike a turnstile and back from whence it came if by doing so the antenna
comes close to resonance'. If that "'stub" is approached before the required
resonant point then the current flow through this stub will be one of two
forms, parallel or series, thus the number of "resonant" frequencies will be
doubled by the addition of this stub. Thus multiple frequencies will occur
which is the object of this particular project. I see little difference to
the approach by that taken by the G5RV where the feed line provides a current
path to obtain specific multiresonant points. The difference in approach by
the poster is that the lines are closed at both ends and to this end I would
suggest the dipole should not be severed at its center but fed in delta
fashion. I have a page where I have done similar work ( combination
loop/dipole ) where the loop is the equivalent of what you are gyrating
towards and this does produce multiresonant points.I might add also that by
providing multi resonant points two mpore differeces will occur when
comparing with the G5RV and that is both reactance AND resistance figures
will gyrate closer to the end required 50 ohm values than the 300 ohm values
of the G5RV which also increases the useable frequency spread. I realise that
the above is contrary to what the resident experts have offerred in response
but that is as I say, is my "two penny worth" If you need me to enlarge on my
thoughts as you approach this project with your son please Email me direct so
that the newsgroup do not have to endure regurgitation E mail address is as
per below. Great post but too many naysayers Have a great week, everybody.
Art KB9MZ Email aun...@ice.net
Richard Ferch
Thanks for your kind words.
Another poster mentioned a 40/15m antenna in the ARRL
Handbook which uses the capacity hat idea. I don't have a copy
of this project, but I'm going to make a guess at how it is
supposed to work, and then make a suggestion for a modified
project.
A 40m antenna is nearly resonant on 15m (third harmonic). The
reason this is only "nearly" is the end effect. A dipole has
to be cut shorter than an exact half wavelength for the
resonant frequency (468/f vs. 492/f), but when it's operated
on an odd harmonic, the end effect is relatively smaller. So,
a dipole cut for 7.0 MHz will also be resonant somewhere
around 21.7 MHz, which is a bit high for 15m (no problem for
tube rigs, but today's radios are a lot fussier). If you
prefer SSB, you might cut your dipole for 7.2 MHz. It would
then have a resonance around 22.3 MHz, which is well outside
the 15m band (but still tantalizingly close).
I believe the idea of the Handbook project is probably to
place moderate-sized capacity hats at or near the 15m voltage
antinodes, around 11 feet from the feed point. It is at least
plausible to suggest that the effect of the capacity hats will
be larger at 15m than at 40m, either because they are
relatively larger (in terms of wavelengths), or because on
15m they are at points of high RF voltage. If either or both
of these explanations holds true, then the 15m resonant
frequency will be reduced relatively more than the 40m
frequency. Even though you may have to shorten the antenna
a bit to bring the primary resonance back to 7 MHz, you may
still be able to bring the third harmonic into the 15m band.
By a bit of fiddling with the overall length and with the
size and/or position of the capacity hats, it may be possible
to make the antenna resonant at the desired frequencies within
both bands.
This brings to mind another possibility. An antenna cut for
3.5 MHz will have resonances somewhere around 10.9, 18.2 and
25.6 MHz. These are not far from the WARC bands. In fact,
the fifth harmonic is probably close enough to use on 17m
without any adjusting. If you want to work 30m, you might try
the same idea to try to lower the third harmonic resonance
to around 10.1 MHz. If the fundamental explanation for the
presumed success with 40m and 15m is the relative size of
the capacity hats at the two frequencies, then by reducing
the 3rd harmonic to reach the 30m band, you will probably
bring the 5th and 7th to frequencies well below the 17m and
12m bands. However, if the explanation is related more to
the position relative to voltage antinodes at the various
frequencies, then it may even be possible to get an antenna
to resonate within the 80, 30, 17 and 12 metre bands (or at
least within three of them). Or, maybe you can shoot for
80, 30, 20 and 15. If you prefer SSB, the starting frequencies
might be around 3.8, 11.8, 19.8 and 27.8 MHz. In that case
you probably wouldn't care about 30m, so you could concentrate
on hitting the 17m and 12m bands.
Does the above suggest a good project for you and your son?
Assuming the effect is real (larger relative reduction of the
higher odd harmonic resonant frequencies compared to the
fundamental), it might also have some slight effect on the
higher resonances of an antenna built to your original
proposal; but remember that those resonances are near 30, 17
and 12m, and not near 40, 20, 15 or 10m.
Whatever you try, I hope you and your son have fun with it.
73, Rich (VE3IAY)