EMRFD Message Archive 5885

Message Date From Subject
5885 2011-03-04 18:25:15 Phil Sittner Off Topic-Antenna end effect/capacative hat
I know this is a bit off topic but it seems to me that capacitive end loading of an antenna is an extension of the "end effect". Can anyone direct me to a definitive source that details the difference/similarity between the two?

Phil kd6rm

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5886 2011-03-06 05:45:59 Tim Re: Off Topic-Antenna end effect/capacative hat
I'm far from a definitive reference but I'm going to pontificate.

"End effect" and "Edge effect" are terms invoked many places in electrostatics and electromagnetism to describe small but noticeable corrections to the simplest calculations, and lumping multiple physical effects into changed constants used in those equations. In terms of dipoles I think "end effect" has historically been used most often to explain why in the real world L=468/f is more correct than the simple L=492/f (and even then it's more complicated because there are reasons other than stray end capacitance why "492" isn't the right number all the time... wire insulation, diameter, ground proximity, and cage dipoles require further tweaks which is not actually just the capactive end loading.)

Today almost everyone has access to NEC-type software that removes the need for the hand-waving about "end effect" that was begun a century ago, but still a good physical understanding of the magntiudes of different effects and when they are enhanced and when they are minimized, is valuable.

In terms of "end effect" vs "capacitive hats", it seems to me that "End Effect" is usually used to refer to the effect of stray capacitance (not designed in) and "Capacitive Hat" is used to refer to purposefully designed in capacitance which often may be at the end (But is not always).

If the Capacitive hat is put at the end it enhances end effect, but they don't have to be at the end. For an example of a "Capacitive Hat" not put at the end, see for example figure 22.13 of the 2008 ARRL Handbook, where the hats are put in the middle of a 40M dipole to change the 15M resonance but not substantially change the 40M resonance.

5887 2011-03-06 07:29:22 KK7B Re: Off Topic-Antenna end effect/capacative hat
That's useful. When considering end effects and capacitive hats I've also found it useful to bear in mind that capacitance is "a region in space where energy is stored in an electric field."

In conventional electronics, capacitors have two leads, but in microwaves and antennas it is common for capacitors to only have one physical connection to the circuit.

Historically, one use of capacitive hats is to increase the current near the top of mobile antennas. The rule of thumb was to make the hat big enough to be unsightly, with unsightly defined by the occupants of the car. Gil published at least one cartoon on the topic of a young lady refusing to enter a car with a large capacitive hat on the mobile antenna. Humor aside, an effective capacitive hat
5888 2011-03-06 07:44:43 Phil Sittner Re: Off Topic-Antenna end effect/capacative hat
Tim, Rick-

Thanks for taking the time to discuss this property. As a young man I befriended a brilliant fellow, Keith Machin (SK),K6WG, who would describe antenna effect by showing a capacitor to ground and would then also explain capacitor hats as having larger surface area and again having greater capacitance to a ground reference. I also remember seeing pictures of Tesla's RF experiments where he would place a large orb atop his antennas. Rick's explanation of energy storage is most helpful.

I'm aware that these properties are modeled in NEC etc but the phenomena is not defined by the program, only the aggregate effect. Would the term "charge" be applicable?

My experiments with these have been conducted using one of DG8SAQ's VNWA's. It's a great tool for many uses.

Again, thanks for the explanations; it gives me something to further explore.

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5889 2011-03-06 09:02:49 Alex P VNA's -- Re: Off Topic-Antenna end effect/capacative hat
Phil and group,

I'm interested in purchasing a VNA and had a look at DG8SAQ's website.

I was wondering if you could take some time and share any unfavorable features or things that could have been done better with this product.

I'll be measuring a variety of components and am interested in obtaining, +/- Jw, real and imaginary, parameters as well as net composite impedance and attenuation.

If anyone else in the group could share their experiences with this and other VNA's that would be very much appreciated.

Hewlitt-Packard-like price points are beyond my budget.

Thanks in advance.

Alex P

5890 2011-03-06 10:15:15 Phil Sittner Re: VNA's -- Re: Off Topic-Antenna end effect/capacative hat

I'm blessed to own both the DG8SAQ and an N2KP. The DG8SAQ has a huge range from 1 khz to over 1200mhz and that was my original motivation for buying it. I had built the W7ZOI spectrum analyzer and needed a method for tuning the 110mhz bandpass filter and this was the perfect solution. When properly configured, you will realize a >-90dB noise floor and this makes it suitable for just about any purpose. The N2KP unit has a -130dB noise floor but is limited to about 60mhz. I suggest you download the helpfile from the DG8SAQ site and review it. You'll find an amazing set of utilities included crystal parameter measurements (entered directly into an excel spread sheet), TDR function, automated network matching, rudimentary spectrum analyzer along with the standard measurements you listed. You'll find the price is right as well. I should also mention that I showed this to a university professor (EE) and he's so impressed that one is
5891 2011-03-07 06:54:27 Alex P VNA's -- Re: Off Topic-Antenna end effect/capacative hat

Thankyou for your info and advice.


5892 2011-03-07 07:30:48 w4zcb Re: VNA's -- Re: Off Topic-Antenna end effect/capacative hat
Phil, your dyslexia is showing. It's an N2PK VNA. The original direct
5893 2011-03-07 16:40:07 Phil Sittner Re: VNA's -- Re: Off Topic-Antenna end effect/capacative hat
I have to admit that you got me there, and I regret the transgression. The N2PK is a delightful instrument and incredibly accurate. I'll try to be a bit more accurate myself in the future.

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