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Author Topic: Can we tune a short dipole with a capacitor?  (Read 3609 times)
JAHAM2BE
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« on: May 05, 2013, 08:12:23 PM »

All designs for shortened dipoles I've seen involve the use of loading coils and finding the correct inductor tap to resonate the dipole at the desired frequency. Fine tuning is done by either adjusting the inductor tap, or by adjusting the capacity hat. For example, the I-PRO Traveller antenna (http://www.proantennas.co.uk/iprotraveller.htm) is a vertical dipole that uses a center loading coil with fixed taps, and allows adjustment of the bottom capacity hat angle to achieve resonance at the desired frequency. N3OX's flex vertical (http://www.n3ox.net/projects/n3oxflex/) also uses fixed coil taps and an adjustable capacity hat for tuning.

Now it seems rather tedious to be adjusting the capacity hat to fine-tune the resonant frequency. My question: is there some way we can use a high-Q variable capacitor (like the ones used in small transmitting loops) to alter the resonant frequency of a short dipole? If so, how would this be done?
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WB6BYU
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« Reply #1 on: May 05, 2013, 10:50:54 PM »

Put it in series with the inductor, and make the inductor larger.  The effective loading
inductance is the combination of the coil and capacitor in series, or basically the difference
in the reactances.

However, this requires a larger inductor than otherwise, which increases losses (the same
resistance for less effective reactance means lower Q.)  To minimize that, you need to use
as large of a capacitor as you can manage to get a low reactance.

Woody Smith W6BCX (inventor of the Half Square and Bobtail Curtain) had an article in
QST where he put a 3-section broadcast variable capacitor (about 1100pf) in series with
his 80m mobile whip with a long plastic extension shaft that terminated with a knob just
behind the driver's seat.  He could reach over his shoulder while he was driving and
adjust the resonant frequency of his antenna:  by tuning it up initially just below the bottom
of the band he could operate all the way to the top of the US band at 4000 kHz.
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JAHAM2BE
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« Reply #2 on: May 06, 2013, 01:47:04 AM »

Put it in series with the inductor, and make the inductor larger.

Good... this actually sounds simple.

You mentioned putting it in "series with the inductor". Could we actually just place it at the feedpoint, since it's still electrically "in series" with the inductor?

A single variable capacitor at the feedpoint would allow that one capacitor to supply the required variable (capacitive) reactance for both dipole legs in a balanced fashion. Any other placement of the variable capacitor (e.g. physically right next to each loading inductor) would require two variable capacitors, one for each leg.

If a single variable capacitor is sufficient, then naturally the next step is to motorize the capacitor for remote control. Within-band tuning could be done remotely with the variable capacitor, while bandswitching would involve changing the loading coil taps and possibly also the matching shunt coil. That is certainly a lot simpler than making a mechanically deformable capacity hat assembly as with N3OX's flex vertical.

---

Edit: some 4nec2 simulation seems to indicate that the tuning capacitance, if placed at the feedpoint segment, doesn't seem to affect the dipole resonant frequency at all. Placing a capacitance in series with each inductance works as expected. So for a dipole, it seems two capacitors would be needed... any way to simplify this to just one capacitor?
« Last Edit: May 06, 2013, 02:45:01 AM by JAHAM2BE » Logged

K3VV
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« Reply #3 on: May 06, 2013, 03:37:19 AM »

Shunt capacitance at the feedpoint has long been used to electrically lengthen small antennas, notably mobile quarter-wave whips.  You can model current distribution on the antenna to see why this is not such a good idea. 

http://en.wikipedia.org/wiki/Electrical_length

See here for a commercial example:

http://www.mfjenterprises.com/pdffiles/MFJ-910.pdf
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N3OX
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« Reply #4 on: May 06, 2013, 06:45:06 AM »

You can put it at the feedpoint. I tuned my 60 foot vertical on 160m with a capacitor. I happened to have a 2000pF vacuum variable which is only 44 ohms reactance when fully meshed. That's a reactance that in my case was less than a 10% increase in the required loading inductance, so I felt it was a reasonable compromise.

Quote
A single variable capacitor at the feedpoint would allow that one capacitor to supply the required variable (capacitive) reactance for both dipole legs in a balanced fashion

That's not really balanced.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #5 on: May 06, 2013, 09:47:54 AM »

Such a dipole was marketed back in the 1980s (?).  I remember seeing it at a convention
around 1984 as a rotatable 80m sloper that was perhaps 20 to 30' long.  I think one of
the guys from Swan was involved in the company.

It had a motor-driven variable capacitor on one side of the feedpoint.
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JAHAM2BE
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« Reply #6 on: May 07, 2013, 06:38:59 AM »

Quote
A single variable capacitor at the feedpoint would allow that one capacitor to supply the required variable (capacitive) reactance for both dipole legs in a balanced fashion

That's not really balanced.

To preserve balance, how about a parallel-fed or series-fed link-coupled tuner at the antenna feedpoint? This would correspond to figures 2 and 3 in the following page:

http://www.amwindow.org/tech/htm/160smallants.htm

Each of these variants would use a single variable capacitor on the antenna side of the tuner. Note that the link-coupled tuner at the feedpoint would be used in addition to the end-inductive-loading and capacitance hats, so that the tuner inductance on the antenna side would be relatively small.

Could such a scheme achieve low-loss, balanced tuning of the dipole (at least, within a single band) using only a single variable capacitor? I am ignoring the variable capacitor on the transmitter side of the tuner, and assume that the link coil can be squeezed or repositioned to achieve a match without needing a variable capacitor on the transmitter side; or, even if a transmitter-side variable capacitor is absolutely necessary, perhaps it would not need to be adjusted within a band?

If this would work, is it correct that the series-fed variant (figure 3) is better (higher efficiency) for a short, end-loaded dipole with capacity hats?
« Last Edit: May 07, 2013, 07:04:05 AM by JAHAM2BE » Logged

WB6BYU
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« Reply #7 on: May 07, 2013, 07:06:59 AM »

Quote from: JAHAM2BE

Could such a scheme achieve low-loss, balanced tuning of the dipole (at least, within a single band) using only a single variable capacitor?



Maybe...   You still have the problem that the capacitor is at a point of maximum current
in the antenna, and loss considerations are similar to when they are used in small loops.
A split-stator capacitor eliminated losses in wiper contacts, but requires twice the plate
area for the same amount of capacitance.  This might not be an issue on 20m and higher.

So you could wind the normal loading coil, except break the wire in the middle and bring
the ends out to the variable capacitor.  By adjusting the turns and coupling on the link
coil you should be able to get a match.


I think the article overstates the efficiency difference between the losses in a link-coupled
tuner (traditionally used with high impedance open wire line) and a T-network tuner
(often used with low impedance coax):  most of the difference is in the impedance they
need to match rather than the circuit itself, and link coupled tuners (including the famed
Johnson Matchbox) have a more limited matching range.  Even open wire loss can have
higher losses than one might expect when used with low impedance loads.  (And expecially
the types with stranded Copperweld conductors when used on 80m and 160m.)



Quote

If this would work, is it correct that the series-fed variant (figure 3) is better (higher efficiency) for a short, end-loaded dipole with capacity hats?




You would certainly want series tuning (used for low impedance loads) rather than parallel
tuning (high impedance).  And I wouldn't use variable coils (especially those with shorted
turns) if you can help it to reduce contact losses.

That said, I don't know that the efficiency will be necessarily better or worse that other
possible matching methods:  that depends on the choice of components, assembly, etc.


An alternate tuning method you might consider is to make a "tuning slug" with some
powdered iron toroid cores and move it mechanically relative to the loading coil.  That
may give you enough tuning range with relatively low losses while avoiding any sliding
contacts.
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JAHAM2BE
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« Reply #8 on: May 08, 2013, 01:53:14 AM »

Quote from: WB6BYU
An alternate tuning method you might consider is to make a "tuning slug" with some
powdered iron toroid cores and move it mechanically relative to the loading coil.  That
may give you enough tuning range with relatively low losses while avoiding any sliding
contacts.
Thanks for the suggestion. Would the slug need to be slid in and out of the coil, or could it simply be moved closer or farther from the exterior of the coil? And if exterior movement is acceptable, should the axis of the motion lie along the solenoid's longitudinal axis, or is movement of the slug perpendicular to the solenoid axis (which might be mechanically easier) acceptable? From my experience "tuning" a homebrew oscillator by moving my hand or a coin near the coil, it seems that movement of the tuning slug outside of and perpendicular to the axis of the coil can achieve tuning, but I'm concerned about if such ad-hoc (de)tuning methods may reduce coil Q appreciably.  

Another complication is that moving a slug in or around the coil may change the coupling coefficient between the loading coil and the link coil. Maybe within a band the slight change in the link coupling coefficient won't cause trouble.
« Last Edit: May 08, 2013, 01:57:28 AM by JAHAM2BE » Logged

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