What causes this?
 

I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.

As I trimmed, I decided to keep track of how
much I trimmed and what the nnn/F number
would be. As I got closer to my goal of 10.15,
the number went down, eventually ending up
at 227.28/10.1955=22.292' Also, the 2:1 swr
bandwidth went up - it started at 567 kc and
ended up at 655 kc.

Either way, I got the antenna up and it's working
fine - I'm just curious why the formula for length
and the bandwidth changed as the antenna got
shorter.

Ken KG0WX

On Wed, 1 Dec 2004 10:58:35 -0600, "Ken Bessler"
wrote:

works out to 229.6 instead of the usual 234/F.

ending up at 227.28

I'm just curious why the formula for length
and the bandwidth changed as the antenna got
shorter.

Hi Ken,

The normal 5% shortening (234/F) is due to what is called "end
effect." Your ends are closer together than for the standard dipole,
and get closer yet when the V is shortened - I suppose.

73's
Richard Clark, KB7QHC

Ken Bessler wrote:

I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.

Just about right for insulated wire. Did you use
insulated wire?
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
...
Ken Bessler wrote:

I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.

Just about right for insulated wire. Did you use
insulated wire?
--
73, Cecil http://www.qsl.net/w5dxp

Nope, I used leftovers from the two 150' rolls I
bought to make my 160m antenna. It's your
standard issue 14/7 stranded bare copper.

Ken KG0WX

Just curious, Ken. Your question seems to be about resonance at
different frequencies. Yet your reported measurements were about SWR.
Are you equating minimum SWR with resonant frequency?

Chuck
NT3G

Ken Bessler wrote:
I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.

As I trimmed, I decided to keep track of how
much I trimmed and what the nnn/F number
would be. As I got closer to my goal of 10.15,
the number went down, eventually ending up
at 227.28/10.1955=22.292' Also, the 2:1 swr
bandwidth went up - it started at 567 kc and
ended up at 655 kc.

Either way, I got the antenna up and it's working
fine - I'm just curious why the formula for length
and the bandwidth changed as the antenna got
shorter.

Ken KG0WX

"chuck" wrote in message
k.net...
Just curious, Ken. Your question seems to be about resonance at different
frequencies. Yet your reported measurements were about SWR. Are you
equating minimum SWR with resonant frequency?

Chuck
NT3G

No. I've been down that road before - an antenna can pose a
200 ohm impedence at resonant frequency resulting in a SWR
of 4:1. I'm simply trying to get the lowest SWR in the middle of
the target range.

Ken KG0WX

Ken Bessler wrote:
I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.

As I trimmed, I decided to keep track of how
much I trimmed and what the nnn/F number
would be. As I got closer to my goal of 10.15,
the number went down, eventually ending up
at 227.28/10.1955=22.292' Also, the 2:1 swr
bandwidth went up - it started at 567 kc and
ended up at 655 kc.

Either way, I got the antenna up and it's working
fine - I'm just curious why the formula for length
and the bandwidth changed as the antenna got
shorter.

Ken KG0WX


234/f is just a starting point.
Dave WD9BDZ

David G. Nagel wrote:
Ken Bessler wrote:
I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.
As I trimmed, I decided to keep track of how
much I trimmed and what the nnn/F number
would be. As I got closer to my goal of 10.15,
the number went down, eventually ending up
at 227.28/10.1955=22.292' Also, the 2:1 swr
bandwidth went up - it started at 567 kc and
ended up at 655 kc.
Either way, I got the antenna up and it's working
fine - I'm just curious why the formula for length
and the bandwidth changed as the antenna got
shorter.
Ken KG0WX

234/f is just a starting point.

The "starting point" in question was a low 40m dipole, strung in the
back alley of the old ARRL HQ building.

For any other antenna, anywhere else in the universe, the "magic number
234" is going to be slightly different.

The difference in SWR bandwidth between 2:1 points is a bit more
complicated, and probably can't be explained in a one-liner. It will be
mostly determined by the interplay between two factors:
1. What the resonant impedance is (in relation to 50 ohms), which
determines the minimum SWR.
2. How quickly the reactive part of the feedpoint impedance changes with
frequency, for different dipole lengths.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

You've gotten a number of good answers, but maybe I can still add a
little helpful information.

The resonant length and the bandwidth of an antenna are determined by
some basic electromagnetic principles. Although simple in concept, the
exact solution for the impedance (and therefore the resonant frequency
and bandwidth) of even an elementary dipole is actually very complex.
The most common method involves solution of a triple integral equation,
which can't be done directly at all, but requires a computer to
numerically approximate the result.(*) The formulas you see in handbooks
are just a rough approximation that's more-or-less good over a limited
range of conditions. The actual resonant frequency and bandwidth are
affected by wire diameter, height above ground, and angle between the
wires, as well as just the wire length. And the relationships aren't
really simple at all.

So the bottom line is that the formulas work well enough to get you into
the ballpark, from which you've usually got to do some trimming -- just
as you did. You can't expect more than that from them.

Readily available, inexpensive or free, computer programs can do the
complex calculations from fundamental electromagnetic principles with
rather astounding accuracy, in a small fraction of a second for a simple
antenna. The computed results can still differ from reality, though, due
to differences between the model antenna and the real one, like nearby
objects or wire insulation not included in the model, wire sag,
capacitance of end insulators, common mode feedline current, and so
forth. But they'll still get you much closer than the simple handbook
formulas. However, the simple formulas and a bit of cut and try are
perfectly adequate for many simple antennas, and might easily be faster
in the long run for someone not familiar with the programs.

(*) Before the ready availability of computers, many different methods
were devised to approximate the solution, with varying degrees of
complexity and accuracy.

Roy Lewallen, W7EL

Ken Bessler wrote:
I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This
gave me a low SWR at 9.5665 mhz which
works out to 229.6 instead of the usual 234/F.

As I trimmed, I decided to keep track of how
much I trimmed and what the nnn/F number
would be. As I got closer to my goal of 10.15,
the number went down, eventually ending up
at 227.28/10.1955=22.292' Also, the 2:1 swr
bandwidth went up - it started at 567 kc and
ended up at 655 kc.

Either way, I got the antenna up and it's working
fine - I'm just curious why the formula for length
and the bandwidth changed as the antenna got
shorter.

Ken KG0WX

Ken, I guess I'm still confused.

As I understand it, one cannot reliably determine the exact resonance of
a dipole by finding the point of minimum SWR. Until this measurement
issue is resolved, there would seem to be little benefit to seeking an
explanation of why the formula appeared not to work.

It is probably too late now, but if you had used an impedance bridge
(MFJ or Autek, for example) you could have found resonance at the point
of zero reactance. All within the limits of the instruments, of course.

Some of the posts suggest other reasons why the formula might not work,
but it is not yet evident to me that it didn't work.

Sorry my earlier post was not more clear. (I'm even sorrier for this
post if you actually used an impedance bridge! Hi.)

73,

Chuck
NT3G