[ARC5] DF-ing.

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: <arc5 at mailman.qth.net>
Sent: Wednesday, July 03, 2013 9:35 AM
Subject: Re: [ARC5] DF-ing.


> 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

     The idea is to resolve the ambiguity as someone else 
stated. Its not necessary for the carioid null to be as good 
as the figure eight null, its only necessary to know which 
of the two opposing nulls gives the correct bearing. There 
is usually a switch to turn the sense antenna on and off. 
Exactly the same arrangement is used on shipboard D/F rigs.
     Anything that disturbs the directional pattern of 
either antenna will cause an error in the null and thus the 
bearing. The loop is usually shielded to prevent its being 
affected by stray fields but its very difficult to provide 
such shielding for the sense antenna.
     The ideal pattern is a cardioid of rotation. The sense 
antenna exactly nulls out one of the figure eight lobes and 
adds to the other. Reception at the sides at the exact point 
of the figure eight nulls is entirely from the sense 
antenna.  Cardioid microphones work on the same principle.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com