Helical Antenna OK for Part15?

[wcummings]

With all of the past discussion about base and center loaded antennas and the assumption that the FCC does not consider the length of the wire in the loading coil but simply its overall length, has anyone considered using a helically wound antenna?

For those who are not familiar with this type of antenna, it is an electrical 1/4 wave antenna constructed of winding 1/2 wave of wire on an insulated pole. (fiberglass, bamboo, treated wood, etc.) Typically there is a capacitance hat placed at the top of the windings. (metal pie plate for example)

The amount of insulated wire needed for 1600 khz would be approximately 292.5 feet.

One could construct a steath antenna by putting heat shrink tubing along the entire length of windings. To a passerby, all that would be apparent would be a 3 metre long pole with a small metal hat.

Would a 3 metre long helically wound antenna pass FCC muster?

 

[radiopilot]

> With all of the past discussion about base and center loaded
> antennas and the assumption that the FCC does not consider
> the length of the wire in the loading coil but simply its
> overall length, has anyone considered using a helically
> wound antenna?
>
> For those who are not familiar with this type of antenna, it
> is an electrical 1/4 wave antenna constructed of winding 1/2
> wave of wire on an insulated pole. (fiberglass, bamboo,
> treated wood, etc.) Typically there is a capacitance hat
> placed at the top of the windings. (metal pie plate for
> example)
>
> The amount of insulated wire needed for 1600 khz would be
> approximately 292.5 feet.
>
> One could construct a steath antenna by putting heat shrink
> tubing along the entire length of windings. To a passerby,
> all that would be apparent would be a 3 metre long pole with
> a small metal hat.
>
> Would a 3 metre long helically wound antenna pass FCC
> muster?
>


I've seen these antennas and they go by the term 'Firestick' and normally are used for the CB band, but I guess you can use it for the AM band if the FCC agent was not on par with this type of antenna... here's a view of this antenna...

http://www.firestik.com/CatalogFrame.htm

Better would be to wind 1/2 wave or 5/8 wave with wire on 1/2 pvc 3 meters long and then cover it with another pvc 3/4" or 1", cap it and your done, but if the FCC ever asked you to show them the antenna... then what?

The 5/8 wave antenna is not going to give you any added benefit as all you are doing is adding more inductance and more resistance to the overall system that you are trying to reduce...

Maybe someone can add to what I've commented.

Radiopilot

 

[rfry]

> With all of the past discussion about base and center loaded
> antennas and the assumption that the FCC does not consider
> the length of the wire in the loading coil but simply its
> overall length, has anyone considered using a helically
> wound antenna?
____________

It doesn't really matter to antenna radiation efficiency if the loading coil is lumped at the base, or distributed along a 3-meter linear distance in the form of a helix.

Below are some numbers for a ground-mounted 3-meter helical with no top loading, self-resonant on ~1.7 MHz, with 10 ohms of ground loss.

For all practical purposes, the radiation efficiency of a helical antenna as described is the same as a conventional ~2.8 meter linear radiator with a base loading coil (other conditions the same).

Height of helical winding, metres .......... 3.00
Diameter of helical coil former, millimetres 50.0
Number of turns on helical winding ......... 845
Wire diameter, millimetres ................. 2.0
Earth electrode loss resistance, ohms ...... 10.0

Quarter-wave resonant at ... 1.697 Megahertz
Helix wire pitch ........... 3.6 millimetres
Length of wire on helix .... 138.1 metres
Total inductance ........... 631.5 microhenries
Coil self capacitance ...... 13.8 picofarads
Total effective capacitance. 13.9 picofarads
Radiation resistance ....... 0.17 ohms, referred to base
Wire HF loss resistance .... 12.18 ohms .. .. ..
Feed-point input resistance 22.35 ohms .. .. ..
Transmission bandwidth ..... 5.6 kilohertz between 3dB points
Power radiating efficiency ... 0.8 percent
Loss relative to ideal 1/4 wave... 21.2 decibels
Base matching capacitor .... 2086 picofarads for 50 ohm match
//