[TowerTalk] Voltage vs Current feed

[RCARIELLO]

Pete,
This is a good question with a very simple answer.
We are dealing with what is known as an alternating current. The amount of
each complete cycle per second determines what our frequency is. A cycle is
measured in degrees and a complete cycle has 360 degrees in it. A cycle has
a physical length that is determined by its frequency or duration in time.
That is why an antenna for 1.8 MHz is longer then one for 28 MHz.
If we measure and mark a wire that is one wavelength long the starting end
will be marked 0 degree point. At the quarter way point marked 90 degrees.
At the half way point marked 180 degrees. At the three quarter point marked
270 degrees. At the end of the wire marked 360 degrees which is the full
wave point. If we had a way to measure what was going on at these five
points we would find that at the 0,180 and 360 degree points the voltage is
the highest then any other point along the wire. We would also find that at
the 90 and 270 degree points the current is the highest then at any other
point on the wire. Where the voltage is highest the current is lowest or
zero, where the current is highest the voltage is lowest or zero.
By feeding this wire at the points that the voltage is the highest
(0,180,360 degrees) this is called a voltage feed. By feeding this wire at
the points that the current is the highest  (90,270 degrees) this is called
a current feed.

As a side note: The maximum antenna radiation is at the highest current
point. Getting the current point away from the ground as in the case of the
ground mounted end fed half wave vertical (voltage feed).This puts the high
current point of the antenna a quarter wave above ground well in the clear.

Rich AA2MF


----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <towertalk@contesting.com>
Sent: Monday, October 15, 2001 1:12 PM
Subject: [TowerTalk] Voltage vs Current feed


>
> OK, here we go with Antennas 101 again.  I've just spent 45 minutes paging
> through the ARRL Handbook (77th ed) and the ARRL Antenna Book (18th ed)
> looking for an explanation of voltage feed versus current feed.  It
> may/must be in there, but I couldn't find an index entry and a search
> through antenna fundamentals chapters turned up nothing.  Yet people who
> know antennas use the terminology all the time.
>
> Can anyone either point me to the discussion I've overlooked in one of
> these books or offer a basic definition of these terms?  Thanks!
>
> 73, Pete N4ZR
>
>
>
>
>
>
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List Sponsored by AN Wireless:  AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!!  http://www.anwireless.com

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