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Author Topic: Rainbowkits sdf1  (Read 390 times)

HAMHOCK75

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Rainbowkits sdf1
« on: October 18, 2020, 11:31:40 PM »

Has anyone tried this?

https://www.rainbowkits.com/kits/sdf-1p.html

It seems like the nearest hardware solution currently available that bears some semblance to the old Ramsey DDF1.

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WB6BYU

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Re: Rainbowkits sdf1
« Reply #1 on: October 19, 2020, 08:51:44 AM »

That’s a very simplistic switched antenna set, which
can be duplicated with a pair of switching diodes and
a 555 timer chip.  It has a bidirectional pattern, which
means it is easy to go the wrong direction.

K0OV describes a mod to make it unidirectional
on his web site (though it was originally developed for
other sets).

The Ramsey DF1 included phase sensing circuitry to
indicate LEFT/RIGHT, which makes a much more
useful circuit.  There are several ways to add this to
a simple switched antenna set, some more complicated
than others, but most don’t require the problematic MF5
audio filter chip that Ramsey used.

I’ve used all these types when I was getting started.
The LEFT / RIGHT indicator is worth the added effort.
All of them get confused in the presence of reflections,
and may give anywhere from 0 to 6 different directions
to the transmitter.  In an open park or empty field they
can work well, but certainly not for hunting across
a metropolitan area - they can send you in circles around
the parking lot at the start.

For the price, the kit may be a good starting point for
those who aren’t equipped to build their own from
scratch, and probably could be upgraded as the builder
desired.

But one should temper their expectations of how effective
it will be, depending on the type of area where they will
be hunting.

HAMHOCK75

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Re: Rainbowkits sdf1
« Reply #2 on: July 02, 2022, 05:13:49 AM »

Thank you for that information. I had a request to use my 3 element 2 M beam to find a bearing but the beam's 3 dB bandwidth is pretty wide so trying to peak the signal on a spectrum analyzer with a preamp was not easy although the signal was sufficiently strong.

I did decide to try the Rainbowkits SDF-1 since the beam eliminated its ambiguity. I found that the null of the SDF-1 was quite a bit sharper than the amplitude peak using the spectrum analyzer so it seems to have served its purpose for not many $.
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W9FR

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Re: Rainbowkits sdf1
« Reply #3 on: July 02, 2022, 08:16:28 AM »

I use the side of my yagi for deep nulls to find noise sources or other signals. Then add or subtract 90 degrees. The sides are very sharp nulls. After determining the null direction, simply look for best signal based upon the F/B of the yagi.  It works for me but YMMV. 73, Noel W9FR
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WB6BYU

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Re: Rainbowkits sdf1
« Reply #4 on: July 02, 2022, 10:39:08 AM »

Quote from: HAMHOCK75

...I had a request to use my 3 element 2 M beam to find a bearing but the beam's 3 dB bandwidth is pretty wide so trying to peak the signal on a spectrum analyzer with a preamp was not easy although the signal was sufficiently strong.



To get a good bearing with a vertically polarized beam (which has
a wider beamwidth than for horizontal polarization), it works best
to have a detector with a more limited range, so it is easy to see
small variations.

A standard FM rig often has a bargraph display with 5 or 7 segments
(even if it is designed to look like more), and a total display range
of about 12 dB or so.  That gives about a 2 dB resolution.  But by
tapping off the S-meter voltage and running it to an external
meter, or, better yet, an audio S-meter that changes pitch with
the input voltage, it is possible to resolve much smaller graduations
to the input signal.

The human ear is very sensitive to pitch.  By using a VCO at audio
frequencies, with the pitch varying with the signal strength, I can
detect differences of 0.25 dB or better using one of my receivers.
That goes a long way to narrowing the accuracy of bearings taken
using a 3-element yagi.  The VCO section of a CD4046 PLL chip
is a simple way to do this.

But that probably is more work than you were planning on...



Quote

I did decide to try the Rainbowkits SDF-1 since the beam eliminated its ambiguity. I found that the null of the SDF-1 was quite a bit sharper than the amplitude peak using the spectrum analyzer so it seems to have served its purpose for not many $.




A null in the pattern can be much sharper than a peak, and the
simple TDOA units do work well in that regard (when clear of
reflections).  That can be accomplished with antennas as well:
WB2HOL's tape measure yagi design was designed with a deep
rear null (at the expense of forward gain) for just that purpose,
and a pair of broadside dipoles fed out of phase (an Adcock antenna),
or a small loop, can accomplish the same thing with no electronics.

In practice, I've found two problems with hunting the null instead
of the peak of the pattern:  it is more susceptible to errors due to
reflections, and with a standard yagi or quad the directions of the
rear nulls can shift with frequency.


In this case, the TDOA did its job, and they are simple to use with
any FM receiver.  Sounds like a good solution!
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