Doppler DF whip length
 

I am building the Ramsey Electronics DDF-1 doppler DF. This uses
four 1/4 wave whip antennas on the corners of a square approximately
1/4 wavelength on a side. These are connected to a rooftop switcher
that activates each antenna in sequence. See:
http://www.ramseyelectronics.com/cgi...ction&key=DDF1

My question is, How dependent is this rig on proper whip length to
function properly? Does sensitivity suffer if my whips are
significantly too short, or does the thing simply stop working
properly?

I want to use it for 6M. I have the room for the 55" antenna spacing,
but my antennas are only around 24" long.

Ken KC2JDY

Ken
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"Ken" wrote in message
...
I am building the Ramsey Electronics DDF-1 doppler DF. This uses
four 1/4 wave whip antennas on the corners of a square approximately
1/4 wavelength on a side. These are connected to a rooftop switcher
that activates each antenna in sequence. See:

http://www.ramseyelectronics.com/cgi...ction&key=DDF1

My question is, How dependent is this rig on proper whip length to
function properly? Does sensitivity suffer if my whips are
significantly too short, or does the thing simply stop working
properly?

I want to use it for 6M. I have the room for the 55" antenna spacing,
but my antennas are only around 24" long.


shorter whips would reduce sensitivity, just as they would for a simple
receiver. the whip spacing can also be reduced, it will just reduce the
direction detection sensitivity as the spacing shrinks. one important thing
is that you don't want to use one that is spaced for 6m on a higher
frequency band... if the spacing gets too much more than 1/4 wave or
approaches 1/2 wave the detector may not respond properly and the direction
indication could be unstable or 180 out from where it should be.

"Ken" wrote in message
...
I am building the Ramsey Electronics DDF-1 doppler DF. This uses
four 1/4 wave whip antennas on the corners of a square approximately
1/4 wavelength on a side. These are connected to a rooftop switcher
that activates each antenna in sequence. See:

http://www.ramseyelectronics.com/cgi...ction&key=DDF1

My question is, How dependent is this rig on proper whip length to
function properly? Does sensitivity suffer if my whips are
significantly too short, or does the thing simply stop working
properly?

I want to use it for 6M. I have the room for the 55" antenna spacing,
but my antennas are only around 24" long.

Ken KC2JDY

Ken
(to reply via email
remove "zz" from address)

Ken,

Very familiar with the "Doppler" type system. You may very well be
better off with non resonant whips. When you have whips near resonance, you
are subject to the phase shift due to small differences due to being
slightly above or below resonance. A test should be to put all the whips
very close together, in a line and receive a signal from broadside (so all
antennas are equidistant from the source...assuming you have a single path
as from a nearby hand held) Then you verify that the pulses out of the
discriminator are non existent.

Shorter whips mean reduced sensitivity. Not all that desirable in a Doppler
since it suffers already. It is nothing other than receiving a 6M signal on
any other 24" whip.

I agree with Steve, both for the reason he mentioned and also because
the unused antennas (at each instant in time) are parasitic elements
to the one in use, and will shift the phase of the signal received in
that active element. To the end of reducing that effect, you should
try to reflect a high impedance to the base of the "off" elements,
because then they won't be resonant. But it would be bad to reflect a
short to the base of the "off" elements, if they are 1/4 wave long.

Cheers,
Tom

"Steve Nosko" wrote in message ...
"Ken" wrote in message
...
I am building the Ramsey Electronics DDF-1 doppler DF. This uses
four 1/4 wave whip antennas on the corners of a square approximately
1/4 wavelength on a side. These are connected to a rooftop switcher
that activates each antenna in sequence. See:

http://www.ramseyelectronics.com/cgi...ction&key=DDF1

My question is, How dependent is this rig on proper whip length to
function properly? Does sensitivity suffer if my whips are
significantly too short, or does the thing simply stop working
properly?

I want to use it for 6M. I have the room for the 55" antenna spacing,
but my antennas are only around 24" long.

Ken KC2JDY

Ken
(to reply via email
remove "zz" from address)

Ken,

Very familiar with the "Doppler" type system. You may very well be
better off with non resonant whips. When you have whips near resonance, you
are subject to the phase shift due to small differences due to being
slightly above or below resonance. A test should be to put all the whips
very close together, in a line and receive a signal from broadside (so all
antennas are equidistant from the source...assuming you have a single path
as from a nearby hand held) Then you verify that the pulses out of the
discriminator are non existent.

Shorter whips mean reduced sensitivity. Not all that desirable in a Doppler
since it suffers already. It is nothing other than receiving a 6M signal on
any other 24" whip.