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Author Topic: How will wire size effect the coil?  (Read 8104 times)

Posts: 33

« on: October 16, 2012, 07:35:04 AM »

I want to construct an off centered loaded short 160m dipole.  I found this on the internet, and it calls for two coils using 80 turns each of 16 gage speaker wire. I have some 18 gage speaker wire and am wondering if I use it instead of the 16g, will it mess things up?  Here are some critical points.

physical length:         48% of a full length half-wave dipole
position of coil:           23% of the distance from center to end insulator
resonant freq. (f):      2023.5 KHZ
inductance (L):          58.7 mh
inductive reactance:  746.5 ohms

Increasing the length of each side of the dipole by 24-inches will bring the resonance down to 1950.0 KHZ, making it suitable for amateur service on 160 meters.

This antenna idea appeared in the April 1990 copy of NAVMARCORMARS Scuttlebutt
Sounds like a possibility for me. Will it work?

Posts: 17479

« Reply #1 on: October 16, 2012, 08:34:33 AM »

This coil calculator is a good place to start, as it is probably the most accurate
one available on the internet:

Try changing the wire size and see what impact it has.  In many coils it doesn't
make a lot of difference, but when a coil is close-wound the self-capacitance
between the turns can become significant.  That is affected by the turns spacing
and the presence of insulation - increasing the spacing between the wire surfaces
by using a smaller wire will add an air gap between the insulation layers, which will
decrease the capacitance between adjacent turns.

Typically the difference is quite small - not more than the other expected variations
due to core material, measurement tolerances, etc., and is easily taken into account
during normal antenna tuning.  But when the self-resonant frequency of a coil is
close to the operating frequency (as can be the case with heavily-loaded antennas)
then it starts to make more of a difference.

Will it work?  Probably so - there is nothing particularly unconventional about that
arrangement as I see it, though the term "off centered" may be confusing.  It looks
like just a shortened dipole with loading coils in each leg.  Such an antenna can
have the loading coils positioned just about anywhere along the wire, though the
required loading inductance changes as you do so.  You can tune it by adjusting
the number of turns on the loading coil or the length of either wire.  Feedpoint
impedance is likely to be well below 50 ohms unless the coils are lossy.

But I'd guess that the inductance is 58.7 microhenries (uH) rather than
millihenries (mH).

If you wind 80 turns of closewound 18 gauge wire instead of 16 gauge the coil will
be slightly shorter, and therefore have higher inductance.  That's probably close
enough, however:  wind the coils, string it up, and check where the minimum SWR
is.  Adjust the length of the wire ends as needed to move it to your favorite frequency.

Posts: 14491

« Reply #2 on: October 16, 2012, 08:42:58 AM »

If you want something commercial, watertight, and ready to go take a look at: shorteners

(Model LC-160)

I'm not sure about the long term survivability of speaker wire out in the weather.

Bob  AA4PB
Garrisonville, VA

Posts: 862

« Reply #3 on: October 16, 2012, 10:40:53 AM »

I want to construct an off centered loaded short 160m dipole.  ...<snip>...

Matched (same physical/electrical characteristics) coils will not work with an off-centered fed doublet (dipole).
If you wanted a shortened dipole for 160M (or any band for that matter), then the best option is a center-fed dipole with a pair loading coils placed a definite distance out from the center (one coil per side, equal distant).

GL, 73, Rich, K3VAT
« Last Edit: October 16, 2012, 10:44:37 AM by K3VAT » Logged

Posts: 33

« Reply #4 on: October 17, 2012, 07:55:24 AM »

Thank you very much for your imformative replies.  And you're right, it is not really an OCF dipole, as the report called it.  Also appreciate the calculaor web page and the commercial site.  73 all  Bernie....WY0Q

Posts: 244

« Reply #5 on: October 17, 2012, 09:03:20 PM »

I used these three pages and found the coil calculator to be very close.    Note on this page the chart showing OD of common PVC pipe when used as coil forms.

A few notes:

I used 14 ga stranded insulated THHN which when close wound (the turns touching) gave a spacing of almost exactly 9 turns per inch.  I entered this figure in the coil calculator and was just about right on.

Any bit that you are off you can compensate for by trimming the ends a bit.

You can reduce overall length of a dipole by up to 50% before you begin losing efficiency.  The use of coils and length reduction comes at the cost of reduced bandwidth.  Here's one already drawn up for you.

If the coils are placed strategically... at about where the ends of a shorter dipole would be, then you can get two bands from it.  For example, you want a shortened 160 m dipole.  At 50% it is the overall length of an 80 meter dipole.  If the coils are halfway out on each leg... they will act like chokes, with the inner segments working 40 meters.  Well, the 40 meter segments will also work 15 meters.  Now you are getting THREE bands from your 160 m coil loaded dipole!

If you put "J-slots" on the ends of your pvc coil forms you can wind more wire on or off the coil from either side, giving a way to adjust some more.  So, first tune the middle segments for 40 meters, winding on a turn or two, or taking off a turn or two to adjust the 40 m band.  When that is working to your satisfaction, adjust the 160 meter band by trimming the ends and/or winding onto or removing turns from the outer ends of the coils.

Since the overall length is about the same as an 80 m dipole, you can run legs in parallel, like a fan dipole, and now have four bands.

Adding some 16.5' legs will give you 20 meters, too.

And a tuner can be used to force 17 meters.  So, that's 160, 80, 40, 20, 17, and 15 meters from your fan dipole.

And I found this, too.

Paul - AE5JU
« Last Edit: October 17, 2012, 09:17:34 PM by AE5JU » Logged

Posts: 723

« Reply #6 on: October 18, 2012, 10:06:07 PM »

The only thing reducing the size of the wire will do, is reduce the current capability of the coil.
The change in inductance can be made up for, by adding more turns to the coils.

N8CMQ   Jeff Retired...
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