[ARDF] 80m antenna patterns: Sense v. No Sense...

[Dale Hunt]

Good explanations so far.  Here are a couple other thoughts...

I think there is a good article on loop and sense antennas on the DARC ARDF
web site, so you might look around there.

Rather than saying that the E and H fields are 90 degrees out of phase, it
is more accurate to say that the signal picked up by the sense antenna and
that from the loop are 90 degrees out of phase.  Given that the two sides
of the loop pattern are 180 degrees apart, shifting the sense antenna by
90 degrees makes it in phase on one side and out of phase on the other.
Most people hunt with the sharp nulls of the bidirectional loop for their
bearings, and just switch in the sense to resolve the 180 degree ambiguity.
In this case the sense tuning only has to be close enough to get perhaps
10dB of F/B - enough to tell which is stronger.

If you are hunting without switching the sense off, the sense tuning is more
critical.  Unless you have whoopee mode in your receiver, the peak is often
too broad to get much of an indication with the receivers most of us use. But 
adding whoopee mode makes it much more practical to hunt the peak.

There are two basic adjustments for the sense antenna:  it has to have the
same output level as the peak of the loop, and the phase shift has to be
exactly 90 degrees.  Many designs use a series coil and resistor on the sense
antenna - the resistor sets the magnitude and the coil is set slightly off
of resonance to give the desired phase shift.  This can be rather difficult,
especially if they aren't set close to begin with.  The old Ron Graham 80m
receiver worked this way, and it was a royal pain to adjust (especially since
the tuning shifted when the cover was replaced.)  Many of the newer designs
(including my QST version) use inductive coupling between the sense antenna
and the loop - this provides the required 90 degree phase shift, so all that
is left is to adjust the signal levels to match.  Much easier!

One of the things explained on the DARC site is that the signal from the
sense antenna changes with height above ground differently than the loop.
This means that the sense antenna tuning depends on how high you hold
the receiver.  So make sure you tune it at the height you plan to hold it.
It may also mean that the sense antenna that is optimum for Bruce in the car
isn't the one that works best when hand-held.

And I think the use of the sense antenna is explained in the ARRL Antenna Book,
or perhaps even the Handbook.  I know when I wrote my article I told the folks
at QST to take a drawing from one of those sources, and they came up with it.

Good luck!

      - Dale   WB6BYU






----- Original Message -----
From: Bruce <brucep at netspace.net.au>
To: ARDF <ardf at kkn.net>
Sent: Thu, 12 May 2011 23:31:37 -0400 (EDT)
Subject: Re: [ARDF] 80m antenna patterns: Sense v. No Sense...

> On 5/12/11 10:14 AM, Matthew Robbins wrote:
>> I'm looking for a simple explanation of how a sense antenna works.
>>
>> Does a simple explanation exist? (It's for a presentation, so words or
>> pictures in any combination are okay.)
>>
>> I gave an ARDF presentation the other day, and, although no one asked me, I
>> was worried someone was going to ask, "How does that work?"
>>
>> My answer was going to be, "I have no idea."  (I have some vague idea that
>> there is an E- and and H-component that are somehow balanced to null one of
>> the lobes and produce a one-peak, one-null cardiod pattern.  Or something.)
>> If there is no other way, even some equations would be okay.
> Nothing special about E and H really.
>
> The two peaks of the loop are out of phase with each other.  The omni
> antenna will be additive with one peak and subtractive with the other
> depending on the wiring of the loop.
>
> The gain of the sense antenna is variable within the front-end of the
> receiver, typically with a trimpot or selected resistor.  It's a
> set-and-forget adjustment, not tweaked during normal operation.  When
> the sense antenna is enabled it is coupled to the receiver input with
> gain equal to that of the loop.  Its signal is equal and opposite of
> that from the out-of-phase side of the loop, which cancels (nulls)
> signals coming from that direction.  It adds to signals coming from the
> in-phase side of the loop resulting in a cardioid pattern.
>
> Matching gain is critical in having an effective sense setup, as is the
> isolation of the sense antenna when it is switched off.
>
Good explanation. I'll add a couple of titbits Matthew casn ignore to 
keep it simple.
I believe the H and E components are 90Deg out of phase, so one of the 
functions of the network at the bottom of the sense antenna is to shift 
an extra 90 Deg so you end up with 180 Deg. I read this somewhere but 
can't recall if it is accurate so maybe someone can confirm ?

Another thing I think handy to think of when you are tuning up these 
things and looking at the pattern (I do this on the foxhunt rotator and 
look at polar plots) is where you are starting from and aiming to get 
to. A bi-directional loop has a figure-8 pattern. As you start to add 
sense with the correct 180Deg, one of the loops of the 8 starts to get 
smaller. The other gets a little larger (Hint: don't mark which way is 
forward till you've tuned the sense!). As you add more sense you get 
something that looks like those fractal patterns with a tiny loop and a 
large loop (slightly squashed circle). Note the big circle is not around 
the origin but mostly off in the 'peak' direction side.
If the sense isn't at the right phase exactly, you end up with skewing 
and not good cancellation. You can tweak the sense tuning a bit, or very 
minor tweaks to the main loop tuning can help by moving around the 
resonance and ending up at a different phase compared to the sense.

As you add more sense the little loop 'turns inside out' and starts to 
move 'inside' the big loop. what you end up with in effect is a null. 
You now have a perfect cardioid pattern. The peak is wide; wider than 
you started with the figure 8 because this circle has 'swallowed' one up 
its rear end.

When using a loop in uni-directional mode, don't rely on the null for 
directions. This is because the sense tuning relys on an exactly 
vertically polarised signal from the transmitter getting to your sense 
antenna. This rarely happens in practice. Eirther the TX isn't vertical, 
or it has come around/over a hill to you and the polaisation has changed 
a bit. So what you end up with is towards that earlier phase of tuning 
above; one big circle and one tiny little one. The null has in fact 
become two nulls each side of the little circle, and these are not 
180Deg away from the peak ! (nor 90deg either...just somewhere vaguely 
backwards, probably asymetric).

Because I foxhunt in the car with a polar plot, I can see the whole 
pattern and make a good guess, but if hand rotated or handheld you can't 
'see' this quickly so you could end up going off at 30Deg to where you 
wanted if you follow a fake null. Many turn off the sense and use the 
nice narrow (and accurate) nulls in the figure-8 to get a good 
direction, but I prefer to just use the peak all the time.


-- 
Cheers,
Bruce

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