Cecil Moore wrote:
Try this test, wire a small 2.5 MHh RF choke across your antenna and
check the before and after noise levels. They will not change.

Obviously a false statement based on limited knowledge and
experience.

When did you do that test?

When did you compare other antennas, like noise on plummer's delight
antennas?

How many Yagi's have floating driven elements without ground paths?

wrote:


How many Yagi's have floating driven elements without ground paths?


None on VHF/UHF/SHF that I build and anyone I know builds. Maybe on HF
it's different. I don't know why it should be, though.

Although normally all the other elements float on anything but 6 meters.

tom
K0TAR

Tom Ring wrote:
wrote:

How many Yagi's have floating driven elements without ground paths?


None on VHF/UHF/SHF that I build and anyone I know builds. Maybe on HF
it's different. I don't know why it should be, though.

Although normally all the other elements float on anything but 6 meters.

You're both over-generalizing. In VHF/UHF/SHF yagi construction it's
quite common to find driven elements connected to the boom or floating.
The same choice or a different choice may be made for the parasitic
elements.

There are many different reasons for these choices, including:
* size of elements and boom
* mechanical strength requirements
* effect of boom on element resonance (depends on mounting method)
* long-term effect stability of element resonance
* feed impedance of driven element (affects choice of feed method)
* number of elements involved
* perceived or actual effects of "static".

This makes the choices quite complex, and makes generalization
impossible. Quote an example, and someone else will quote you a
counter-example.

Just one other thing: PLEASE let's not talk about elements being
"grounded", way up there at the top of a tower.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:

This makes the choices quite complex, and makes generalization
impossible. Quote an example, and someone else will quote you a
counter-example.

Just one other thing: PLEASE let's not talk about elements being
"grounded", way up there at the top of a tower.

The entire topic and people's reactions about precepitation noise is
fascinating.

People actually seem to think the little particles voving through the
air charge the antenna to a different potential than the air around the
antenna, and that a "dc" path to earth or "dc" path around an element
somehow magically stops RF noise.

Certainly it is possible for a conductor very well insulated from earth
to charge to the potential of the static field around the antenna, and
flash over to "ground" through some path that breaks down. Of course
only a fool would have equipment that allows that to happen while large
antennas are being used or connected. This is an entirely different
thing than the corona discharge that makes what people typically call
"P-static".

73 Tom

wrote:
People actually seem to think the little particles voving through the
air charge the antenna to a different potential than the air around the
antenna, and that a "dc" path to earth or "dc" path around an element
somehow magically stops RF noise.

Please don't accuse people of believing in magic until you
understand the physics involved.

There's no magic involved. Dry-air dust and snow particles
certainly can transfer a charge to an uninsulated wire in
the air. It is simple physics to realize that when a highly
charged particle touches a conductor, the charges between
the two objects will be equalized. It would be magic if the
particle and the wire did NOT equalize their charges by
transferring excess differential charge when they come
into contact.

Given a dipole where one element has a DC path to ground
and the other element is floating with respect to ground,
it is reasonable to expect the charge between the two
elements to be different and they often are. The difference
in charge will build up to the point where the smallest gap
between the two conductors arcs. In my configuration in
the Arizona desert, that gap existed at the coax connector
at the transceiver and it did indeed arc badly. It woke me
up and scorched my rug after I disconnected it from my
transceiver. The arc was bright enough to light up the
room and certainly caused RF noise. The wind was blowing
around 30 mph and there wasn't a cloud in the sky.

Given an extreme relative charge between the element with
a DC path to ground and the element without a DC path to
ground, anything that will bleed off that charge will cure
the problem. An RF choke works. A 4:1 voltage balun works.
A resistor works.

Perhaps we can convince a desert ham to hang an o'scope
on a clear-sky, dry-air system on a windy day and report
back here with some results. Many other hams have reported
the same thing during a dry-air snow storm.
--
73, Cecil http://www.qsl.net/w5dxp

wrote:
People actually seem to think the little particles voving through the
air charge the antenna to a different potential than the air around the
antenna, and that a "dc" path to earth or "dc" path around an element
somehow magically stops RF noise.

Let's say we have the following bare-wire dipole system
link coupled to the receiver (in fixed font).

|
| dipole element A link coupled
| -////-
+--------------------------------------------------///////--+
|
+-----------------------------------------------------------+
|
| dipole element B
|

Let's assume one highly charged particle hits dipole element
A and transfers half of its charge. Element A will then have
an excess charge one half of which will migrate through the
link while equalizing the charge between element A and element
B. That charge migration/equalization no doubt results in an
RF noise pulse.

This experiment can easily be performed by anyone.
Rub one's leather soles on a wool carpet, touch one side of
the dipole, and listen for noise in the receiver. Guaranteed,
it will be there.

Now multiply the above by millions of charged particles
randomly encountering the bare-wire dipole. The charge on
each side of the dipole will never be exactly equal. Thus,
continuous broad-band noise will be continuously transferred
through the link as long as the particles are transferring
charge to the antenna. That's what some hams are hearing
during dry-air dust and snow conditions. Some have even reported
being able to hear individual particle collisions from large
snowflakes.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
wrote:

Your argument makes perfect sense. Cecil, I was suprised that you actually had arcing to the point of scorched carpet. What voltage do you think this potential represents? Is this a relatively constant voltage. By that, I mean is there usually SOME voltage difference during days or weeks? My naieve question is: Could this serve as a trickle charge for a battery when not used for RX/TX?

Ian White GM3SEK wrote:
This makes the choices quite complex, and makes generalization
impossible. Quote an example, and someone else will quote you a
counter-example.

Seems the assertion should be that precipitation noise
can exist on some antennas but not on others.

Just one other thing: PLEASE let's not talk about elements being
"grounded", way up there at the top of a tower.

How about "DC grounded"?
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Ian White GM3SEK wrote:
This makes the choices quite complex, and makes generalization
impossible. Quote an example, and someone else will quote you a
counter-example.

Seems the assertion should be that precipitation noise
can exist on some antennas but not on others.

Living in this climate, I don't have any particular view or experience
about static noise (but ironically we're just about to get one of our
few thunderstorms of the year... better go and do something about that).

Just one other thing: PLEASE let's not talk about elements being
"grounded", way up there at the top of a tower.

How about "DC grounded"?

By all means; so long as we never forget to include the "DC" part.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:

Tom Ring wrote:

wrote:

How many Yagi's have floating driven elements without ground paths?


None on VHF/UHF/SHF that I build and anyone I know builds. Maybe on
HF it's different. I don't know why it should be, though.

Although normally all the other elements float on anything but 6 meters.


You're both over-generalizing. In VHF/UHF/SHF yagi construction it's
quite common to find driven elements connected to the boom or floating.
The same choice or a different choice may be made for the parasitic
elements.

There are many different reasons for these choices, including:
* size of elements and boom
* mechanical strength requirements
* effect of boom on element resonance (depends on mounting method)
* long-term effect stability of element resonance
* feed impedance of driven element (affects choice of feed method)
* number of elements involved
* perceived or actual effects of "static".

This makes the choices quite complex, and makes generalization
impossible. Quote an example, and someone else will quote you a
counter-example.

Just one other thing: PLEASE let's not talk about elements being
"grounded", way up there at the top of a tower.




Maybe I should have defined what I meant when I said floating. I think
we have been talking about a DC grounded element, and that's what I
meant with respect to floating.

Virtually all of the homebrew yagis that I have seen, and most of the
commercial are gamma match, or T match. The gamma is not going to carry
the ground through, but is used almost exclusively on 6 meters where
driven elements are rarely insulated from the boom. 2 through 432 are T
matches and that DC grounds the driven element regardless of how it's
mounted. Going up we see mostly loop yagis, and I've never seen one of
those that's not grounded. I know there are regular (non-loop) yagis
used 902 and up, but I never seen one to see how they are driven.

So, for me anyway, the generalization works.

And I'm just watching Silverstone, Ian, so don't tell me how it went!

tom
K0TAR