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Author Topic: Spiral capacity hats?  (Read 1412 times)

Posts: 277


« on: April 03, 2014, 09:23:26 PM »

I have a shortened vertical dipole antenna that uses loading coils and large capacity hats. It uses a shunt coil (beta match) at the feedpoint to match to 50 ohms. The antenna looks like this:

I was considering an experiment to remove the loading coils and replace the capacity hats with spiral coils (also called pancake coils), similar to a Petlowany dipole or a TAK-tenna. I would connect the end of each aluminum radiator to a wander lead (alligator clip) that would then be used to select a tap point on the pancake coil. I would tap the coils symmetrically so each side of the dipole is a mirror image of the other.

My hope is that this might simplify the mechanical construction as the spiral coil might provide both inductive loading and capacitive end loading at the same time. Note that I want to use the dipole as a dipole, meaning that unlike the TAK-tenna I am using a balun and do not want the coax to radiate.

My questions:

1. The current antenna can tune up from 40m to 20m just by tapping the coils symmetrically about the feedpoint. If I switch to using pancake coils, is it realistic to expect to be able to tune up on several bands just by selecting taps? The complicated thing is that I'm altering both the end capacitance and the end inductance with the tapping, there also seems to be a "transmission line stub inductor" (,66072.msg430844.html#msg430844) within the spiral that needs to be taken into account. Basically, I'm not really sure how these pancake coils will behave when incrementally tapped at different positions.

2. With pancake coils, is a beta match (shunt coil) still appropriate to use for matching this antenna to 50 ohms?

3. Is it possible for the pancake coils to be as efficient as the current solution of the lumped inductor + capacity hat?

4. Any recommendations on the spiral coil geometry? (Wire spacing, etc.)

Thanks for any advice.


Posts: 934

« Reply #1 on: April 04, 2014, 01:58:22 AM »

Read this months QEX March/April  "Actual Measured Performance  of short Loaded ANtennas" W9UCW. It will  be very interesting information for you  that is supported by field strength measurement. There is a lot of data on capacity hats and their placement.

Posts: 709

« Reply #2 on: April 04, 2014, 05:47:20 AM »

Jerry Sevick, W2FMI wrote [1] "... my final design was for a six-foot antenna for 40M, which included a high degree of top hat loading and gave the highest radiation resistance.  The final structure was a breach umbrella antenna [design] ..."  It appears that Sevick determined that the optimum capacity hat would be  what you have as your posted photo, i.e., multiple radial lengths.  I'm aware of no other literature where the use of spiral capacity hats was shown to be even closely similar, performance-wise, to radial capacity hats (for verticals or for dipoles).

Others may comment on the physics involved and why this is so.

73, Rich, K3VAT

[1] The Short Vertical Antenna and Ground Radial, CQ Communications, 2003, p vi


Posts: 709

« Reply #3 on: April 04, 2014, 06:09:32 AM »

Also ... Regarding 'workability' of you antenna system, and by that I mean how well does this antenna hear, how well does it propagate HF radiation, is governed or determined much more by the antenna's near-field environment (given that the antenna is of proper design & construction and properly fed).  It is well-known that any antenna works best when placed well out in the open, away from nearby obstacles, especially those made of metal.  When I took my attic-based simple monoband dipole and relocated it out in the back yard ~100 feet from the house at 25' high I was amazed by the performance difference.

Unfortunately, many location-compromised amateurs are forced to use indoor antenna systems or antennas in very close proximity to homes/apartments, but there have been some clever innovations to try and mitigate these limitations including full-size, stealth wires hung from balconies or strung between ends of adjoining apartments.  I believe that there are numerous posting here on eham that describe systems using these techniques.  Perhaps this (going stealth) is something that you might (re)consider and others may have even better remedies (unless this is kinda like a research project for you).

73, Rich, K3VAT
« Last Edit: April 04, 2014, 06:12:00 AM by K3VAT » Logged

Posts: 2367

« Reply #4 on: April 04, 2014, 10:58:05 AM »

Let's recall the basics:

-A capacitive hat is.....A Capacitor

-A flat plate provides maximum capacitance

-A Capacitive had made with radials and/or perimeter wire is an approximation of a flat disc with windows removed.

-A spiral is a flat coil

-A spiral flat coil with a single electrical connection is a poor approximation of a radial & perimeter device which is an approximation of a flat plate!

Posts: 1724

« Reply #5 on: April 04, 2014, 12:48:28 PM »

If one could re shape a thin solid disc of proper size for example a 90 degree bend relative to the center of the disc and press it into the point where the top of the wall meets the ceiling the maximum capacitance can be realized.

This would work for an indoor version where the antenna is stood off the wall.

I will build this for the top tip of my vertical short dipole and do the exact thing on the bottom tip

I do not know why I did not think of that sooner.

This will raise the Rrad up a little.
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