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[KI4OYV]

K7FF & SOFAR,

Just my opinion and like a-- h---- everyone has one..
Maybe this is the reason that we're losing people from the hobbie. Both of you sound like teenage girls. Stop the FLAMING! Does no good for the rest of us who want to enjoy the hobbie. Reading from other forums that SOFAR posts to, seems like he's the instigator who loves to start these flaming wars. 

[KH6AQ]

I have used schedule 40 white PVC pipe for antenna coils that were out in the weather (and sun). After two years the PVC looked as good as the day it was installed; no deterioration.

[K7FF]

I have used schedule 40 white PVC pipe for antenna coils that were out in the weather (and sun). After two years the PVC looked as good as the day it was installed; no deterioration.

It's rarely wise,
I must advise,
To post a disagree,
To the dog a-alpha flea,
Or, you'll surely push up daisies, post sunrise.

[KH6AQ]

It was stated that because the Superantenna loading coil is wound on (plasticized) PVC the losses are "excessive." Let's quantify the loss. Running 100 watts in a typical installation at 7MHz I estimate less than 5 watts of loss in the coil PVC form.

Disclaimer: This is a a ball park estimate based on some simplifications, such as the coil current being the same at all points and the capacitance being calculated for an infinite block.



The antenna resonates at 7 MHz using a 6' whip. Using EZNEC we see that the loading coil reactance is 1400 ohms.

From the Superantenna photo I estimate the number of coil turns is 60 and the diameter is 1.5 inches.

For system losses of 25 ohms the coil current at 100 watt is 2 amps.

The voltage across the coil is 2A x 1400 ohms = 2800V

The maximum voltage between adjacent turns is 2800V/60T = 47V. The average voltage is 23.5V. I'll use 0.7 x 47V = 33V so the loss works out correctly.

The capacitance between adjacent turns is less than 20pF. That is if the coil is embedded in a block of plasticized PVC. Let's calculate the losses worse-case and call it 20pF. If anyone is interested I'll post the method to calculate the capacitance.

The capacitive reactance between adjacent turns is 1140 ohms.

The displacement current between adjacent turns is 33V/1140 ohms = 29mA

The dielectric loss resistance between adjacent turns is 1140 ohms X 0.1 = 114 ohms. The loss tangent of 0.1 at 7MHz is interpolated from a paper by G3YNH.

The power loss between adjacent turns is (29mA)^2 x 114 ohms = 96mW

The total dielectric loss is 96mW x 60 turns = 5.8 watts

Because some of the displacement current goes through air the actual dielectric loss is less than 5.8 watts. Let's be generous and call it 5 watts.


Links:

http://g3ynh.info/zdocs/comps/part_6.html


http://newsuperantenna.com/MP1-SuperStick-Portable-Vertical-Antenna-Deluxe/

[KH6AQ]

How much of an improvement would we get by switching from PVC to Teflon? The coil dielectric loss would go from 5 watts to virtually zero.

In this example the radiation resistance is 1 ohm and the system loss resistance is 25 ohms giving a radiation efficiency of 3.85%. It radiates 3.85 watts for 100 applied. The coil dielectric loss looks like 1.3 ohms of series resistance in series with 23.7 ohms of other losses. Switch the coil form to Teflon and the system losses go from 25 ohms to 23.7 ohms. The radiation efficiency is now 4.04% for a gain increase of 0.2dB.

The decrease in coil current due to the decreased capacitance is around 0.5% leading to around 0.5 watts less loss. The total difference is still about 0.2dB.

Where the difference matters is in the power capability of the antenna. It might increase by 35%.

[K7FF]

WX7G:

Splendid!

While I am not competent to comment on the specifics and mathematics of your dissertation, I can readily commend and appreciate its conclusion(s).  To quote the Master Barb, trashing PVC (perhaps literally) is really 'much ado about nothing'.

PVC detractors will claim that it is just a pile of 'crap', without any logic or proof in support.  The babblings of mindless fools.

It was quite refreshing to read your presentation this Sunday morning, and I shall be looking forward to more.

Derek
K7FF

[K0BG]

PVC is fine when it's new. Unfortunately, is ages poorly in the sun. As it does, it dielectric constant deteriorates. Chances are you won't notice any change unless you you have a Megger or other instrument to check it.

[KQ6XA]

The Super Antenna MP1C coil form is not made of PVC.
It is made of a low loss RF material that is ultraviolet resistant.

[W9IQ]

The Super Antenna MP1C coil form is not made of PVC.
It is made of a low loss RF material that is ultraviolet resistant.

You are responding to a 5 year old thread but now you have my curiosity piqued. What is the polymer?

- Glenn W9IQ

[LYFAN]

Superantennas changed the material of the coil form, sometime between the 2016 posts and 2020, fwiw.