[ARC5] DF-ing.

[Jim Wiley]

In general terms, adding a "sense" antenna to a loop changes the 
response from a "figure-8" to a "cardioid" pattern.  The depth of the 
null in the cardioid is dependent primarily on the balance of signal 
delivered to the receiver from each antenna.  If the signals are 
precisely of the same strength, then a good cardioid with a sharp null 
will and a broad peak 180 degrees from the single null be the result. 
However, if the balance is not correct, the depth of the null will 
suffer.  The usage of the DF antenna thus changes from one where a 
relatively sharp null is the desired response, even though that null is 
bidirectional, to one where the antenna can be used to look for a much 
broader "peak", or signal maximum.

Optimum DF capability comes from using both patterns, using the peaking 
pattern to determine the general direction of the signal, and the 
figure-8 null to get a more precise determination of the exact 
direction.   All this assumes only one antenna, either a loop or a loop 
plus sense antenna is used. If a more complex antenna is used, then 
different techniques are possible.

At VHF and UHF frequencies, a 4-element square array of monopoles or 
dipoles  is small enough to fit on a vehicle roof.or short mast.  Such 
antennas are electronically rotated at high speed, and can give 1-degree 
resolution of the direction of the incoming signal, less any ambiguities 
caused by reception of signals from reflected from other objects.  Using 
such a system in a moving vehicle tends to make it possible to determine 
the "true" direction by averaging out the error signals, sort of like 
separating wheat from chaff.

Other techniques can be used to give reasonably precise bearings at HF, 
for example by using crossed ferrite core loop antennas and an 
electronic driving signal to create the effect of the 4-dipole array 
used at VHF.  An example is the common ADF found in many light aircraft 
- or at least once common - it is being obsoleted by GPS.  Up in these 
parts, most pilots I know (meaning the ones still alive) insist on 
retaining the ADF in their aircraft.  The time honored "belt and 
suspenders" approach.

- Jim, KL7CC


On 7/3/2013 8:35 AM, Kenneth G. Gordon wrote:
> On 3 Jul 2013 at 7:25, J. Forster wrote:
>
>> My understanding of the pattern of a loop alone in free space is a
>> figure-8 with both nulls in the plane of the loop.
> Correct.
>
>> Adding in the sense antenna, degrades one of those nulls, (and
>> possibly improves the depth of the other) by adding in a roughly
>> omnidirectional signal.
> Yes, to both statements.
>
> However, as Mike points out, there are many, many factors that can degrade
> that null.
>
> Supposedly, the "perfect" result of adding a properly-phased signal from the
> "sense" antenna is to completely eliminate one null of the figure-8, and at the
> same time, enhance the other one, i.e., making it deeper and sharper.
>
> However, actually achieving that "perfect" result is practically impossible in
> normal use. Most of the time we have to settle for "good enough".  And
> sometimes, given external factors, that "good enough", isn't.
>
> Ken W7EKB
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>