[GreenKeys] On PVC poles

[Richard Knoppow]

    I have lost something in this thread. It appears to be about 
the effect on the loss of a coil when winding it on a plastic 
former. In fact, even at communications frequencies, that is 
30Mhz and below, the coil former has an definite effect on Q. For 
frequencies above about 10Mhz a very low loss material like 
polystyrene is necessary. There may be other plastics with 
similarly low loses.
    Keep in mind that the loss is not just a matter of some power 
loss but of the effect on the Q of the inductor. If the reactance 
and resonance is important to the function of the antenna the Q 
may be critical and losses in the former are important.
    There are charts showing losses of various materials on the 
web. Make sure the loss is measured at the range of frequencies 
you are going to use. A Q meter is very helpful with practical 
materials.

On 7/17/2020 6:21 AM, Bob kb8tq wrote:
> Hi
>
> What frequency(s) is the antenna targeted at? For < 30 MHz, just about anything
> that is “plastic” will be reasonably low loss. Consider that a lot of people have
> trapped antennas with trap Q’s around 10 to 20. They seem to work fine.
>
> If you are building something for UHF then indeed the “microwave test” makes
> a lot of sense.
>
> =====
>
> When you do the “coil” part of the helical, turn spacing is one of the variables.
> You may find that a wide spaced coil works better than a close wound one.
>
> Bob
>
>> On Jul 16, 2020, at 11:50 PM, Dave Horsfall <dave at horsfall.org> wrote:
>>
>> (Somewhat on topic, as I did use "glass" RTTY[*] some years ago, and am trying to find room for a real one).
>>
>> I have a need to make a helical (easily concealed etc), but I am somewhat baffled by "common knowledge" so I figured that this list is the best resource :-)  A magnetic loop is too directional for my tastes, as is also far too high-Q i.e. needs to be re-tuned if you shift by a just a few kHz.
>>
>> Metal is obviously right out, and I understand so is wood (because of its water content), so I thought about a length of PVC pipe with the helical wound around it.
>>
>> Now, herein lies the tale...
>>
>> Apparently PVC pipe may or may not contain carbon, and the only way to find out (apart from deciphering the manufacturer's code) is to place piece of it inside a microwave oven (with the wife not looking) with a glass of water to act as a dummy load to protect the magnetron.
>>
>> If the PVC is warm, it contains carbon and is therefore unsuitable...
>>
>> So, what is the veracity of this claim?
>>
>> [*]
>> And yes, I know about the CR/LF problem, as with 72 columns.
>>
>> -- Dave VK2KFU
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL