80 m short end feed
 

The article in the May 2004 issue of QST on ppg 28. It is for a shortened end
fed 1/2 wave 80 meter antenna. How well does this antenna perform.

Will it be an effective multiband antenna for 80m - 10m? I have no way of
modeling the antenna and am not sure how to model it. Can somebody model it
and let me know how it works as a multiband? Will performance change if it is
made into an inverted "L" with the vertical section about 10-20ft high and the
horizontal section about 20' off the ground. I am interested in NVIS on 80/40
and dx on the higher bands.

Randy KA4NMA

Theplanters95 wrote:
The article in the May 2004 issue of QST on ppg 28. It is for a shortened end
fed 1/2 wave 80 meter antenna. How well does this antenna perform.
Will it be an effective multiband antenna for 80m - 10m?

Here is a rule of thumb that you can take to the bank. If an shortened
80m antenna has loading coils, it will not perform well on the higher
bands. Why? XL=2*pi*f*L - do the math. XL *increases* proportional
to frequency. For antenna efficiency, you NEVER want XL to be more
than you need. And there's something called self-resonance where
XL=self-XC, a terribly lossy condition where the coil current is
sky high.

This is one of tricks that Mother Nature plays on us hams. XL goes
the "wrong" way with frequency but (pardon my Russian) "tough ****sky",
that's the only way it goes.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Theplanters95 wrote:
The article in the May 2004 issue of QST on ppg 28. It is for a shortened end
fed 1/2 wave 80 meter antenna. How well does this antenna perform.
Will it be an effective multiband antenna for 80m - 10m?

Here is a rule of thumb that you can take to the bank. If an shortened
80m antenna has loading coils, it will not perform well on the higher
bands. Why? XL=2*pi*f*L - do the math. XL *increases* proportional
to frequency. For antenna efficiency, you NEVER want XL to be more
than you need. And there's something called self-resonance where
XL=self-XC, a terribly lossy condition where the coil current is
sky high.

This is one of tricks that Mother Nature plays on us hams. XL goes
the "wrong" way with frequency but (pardon my Russian) "tough ****sky",
that's the only way it goes.

Could a loading coil be used more like a trap?

73, ac6xg

Hi Cecil,

Would the coil act as a trap and only the first 32 ft be used on the other
bands?

Cecil Moore wrote:

Jim Kelley wrote:

Cecil Moore wrote:
This is one of tricks that Mother Nature plays on us hams. XL goes
the "wrong" way with frequency but (pardon my Russian) "tough ****sky",
that's the only way it goes.

Could a loading coil be used more like a trap?

Traps are a different subject. Self-resonant coils should only
be used at a current minimum point. IMO, they are much too lossy
to be used at a current maximum point.

But a trap needn't be any more self-resonant and lossy that a loading
coil. A loading coil could function as both if it were in the proper
position for the upper band(s). No?

73, Jim AC6XG

Jim Kelley wrote:

Cecil Moore wrote:
This is one of tricks that Mother Nature plays on us hams. XL goes
the "wrong" way with frequency but (pardon my Russian) "tough ****sky",
that's the only way it goes.

Could a loading coil be used more like a trap?

Traps are a different subject. Self-resonant coils should only
be used at a current minimum point. IMO, they are much too lossy
to be used at a current maximum point.
--
73, Cecil http://www.qsl.net/w5dxp



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Jim Kelley wrote:

Cecil Moore wrote:
Traps are a different subject. Self-resonant coils should only
be used at a current minimum point. IMO, they are much too lossy
to be used at a current maximum point.

But a trap needn't be any more self-resonant and lossy that a loading
coil. A loading coil could function as both if it were in the proper
position for the upper band(s). No?

I'm not sure about "both". It's more than obvious that a resonant
coil+cap will (almost) always be more efficient than a self-resonant
coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Jim Kelley wrote:

Cecil Moore wrote:
Traps are a different subject. Self-resonant coils should only
be used at a current minimum point. IMO, they are much too lossy
to be used at a current maximum point.

But a trap needn't be any more self-resonant and lossy that a loading
coil. A loading coil could function as both if it were in the proper
position for the upper band(s). No?

I'm not sure about "both".

What would it take to get you to be sure about it?

It's more than obvious that a resonant
coil+cap will (almost) always be more efficient than a self-resonant
coil.

And I though it more than obvious that a series inductor could function
as a low pass filter.

73, Jim ac6xg

Could a loading coil be used more like a trap?

Traps are a different subject. Self-resonant coils should only
be used at a current minimum point. IMO, they are much too lossy
to be used at a current maximum point.

IMHO a trap behaves like a disconnection (high series impedance) at
its resonant frequency, effectively shortening the antenna wire. It
behaves as a loading coil at any lower frequency. For example a
trapped 40 + 80 metres dipole is shorter than a full-size single-band
80 metres one due to the loading effect of the traps. The losses in a
trap on LOWER THAN RESONANT frequencies are an issue, of course - the
trap is placed off a minimum current point at these frequencies !
Maybe coax traps (e.g. http://members.shaw.ca/ve6yp/CoaxTrap.html)
would be fine ?

73 Ivan OK1SIP

Jim Kelley wrote:
What would it take to get you to be sure about it?

If "loading coil" and "trap" had the same definition?

And I thought it more than obvious that a series inductor could function
as a low pass filter.

My point exactly. Hence the previous rule of thumb
stated two different ways.

1. Don't use a 75m loading coil while operating on
higher frequencies.

2. Don't use a low-pass filter on *operating* frequencies
above its passband.
--
73, Cecil http://www.qsl.net/w5dxp



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