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Author Topic: 2m DF Loop with Integral Sense Antenna - Leaving excess braid on?  (Read 15442 times)

Posts: 551


« on: January 24, 2015, 09:14:18 AM »

Im working on a 145MHz RDF Loop antenna, based on a design I found on the 'net which comprises a 1/2w loop made from coax, with the braid forming a 1/4w 'sense' antenna, and also acting as a stub to match the antenna. In this design the excess braid is of course removed from the loop.

My question is, if the excess braid is electrically (at least at dc!) disconnected, in other words it is just a length of copper tubing over the loop with no connection either to the inner conductor, ground, the 1/4w stub or anything else, would it effectively be 'ignored' by the RF, or will it still influence the antenna?

The reason i'd like to know, is that im intending building my loop out of a 1/2w length of half-inch Heliax, with the handle and feedline being made from a section of 1 1/2" Cellflex. The antenna connector, a BNC socket, being directly installed into the solid conductors of the Cellflex handle at the base. In this design, removing the excess outer conductor, as opposed to just cutting it and having it disconnected, will be quite a long and tedious task, and will also mean I have to use quite a bit of heatshrink tubing to seal the antenna!


Posts: 15438

« Reply #1 on: January 24, 2015, 05:00:15 PM »

Is that the design that appeared in Technical Topics some years
back, perhaps a Canadian design?  I remember reading some
feedback that it was difficult to duplicate and get working.

Whether you can get by with leaving the shield on one side would
depend on how the sense antenna was coupled into the circuit, and
I can't remember the details of the article. The original problem may
have been a lack of balance between the two sides of the loop, in
which case leaving the shield on could help, but again it depends
on the details.   Might be worthwhile testing the idea with regular
coax first before building it with Heliax.

Posts: 27

« Reply #2 on: January 25, 2015, 09:11:37 PM »

Not sure what you have. Something to check out is an electrician's BX cable cutter. It looks like a fancy can opener, and is used for splitting open flexible armor electrical cable. Sure beats a hack saw.

Posts: 15438

« Reply #3 on: January 27, 2015, 10:07:11 PM »

Ok, I found the articles in the RSGB Technical Topics Scrapbook, volume II.  (I think - I don't have it in front of me.). One described the original design and a second was feedback and improvements from some who had tried to build it.

The design is narrow-band, with no provision for tuning.  There is some quirky stuff happening to get the required 90 degree phase shift on the sense antenna, which uses the outside of the coax as a counterpoise (so an effective balun seems to prevent the unit from working.)

A half wave loop normally would have a very high impedance, but the part covered with braid is a short-circuited quarter wave, so the antenna is effectively open circuited at the top, using the coax inner for half the loop and the braid as the other half.

One comment also said that cheap coax with poor braid coverage didn't work.

It looks to me as though you could split the center of the outer shield and feed the two sides as the loop, or perhaps ground both bottom ends and use the inner as a coupling link.  But that doesn't provide the sense antenna for a unidirectional pattern. Based on my experiments on 80m, using transformer coupling is a much more reliable (and wideband) approach to get the required 90 degree phase shift for the sense antenna.

Posts: 551


« Reply #4 on: January 28, 2015, 03:47:30 AM »

It sounds like the same one, I only have the info found here

However, due to certain construction difficulties ive decided to abandon this approach in favour of a 'Tape Measure' antenna,

I have in fact built the loop, but testing it is another issue, due to the difficulty of getting a feedline connected!

Details of my build can be found on my blog


Posts: 15438

« Reply #5 on: January 28, 2015, 08:31:11 AM »

Yes, that's the first of the two articles from G3VA's Technical Topics column in RADCOM.
(Including a reference to Peter G3RZP, who frequents these boards.)  The second was a month or
two later and included comments from those who tried to build it.

I think there are likely some better loop antenna designs available, as well as other methods for
achieving a null (phased dipoles, ZL Special, Moxon beam, etc.)

The standard WB2HOL "tape measure yagi" is designed for a very sharp rear null at 146.56 MHz, at
the expense of a bit of forward gain.  Hunting the null is capable of very sharp bearings, and can
be especially useful when you don't have a good attenuator, as it is easier to see the difference of
a 30dB null on a standard FM HT than a 3dB peak in the pattern.  However, getting a reliable null
that holds over frequency is not trivial:  a pair of phased dipoles fed with equal-length cables will
work, as the null is not frequency-specific, as will a standard loop, but both are still bidirectional.

I hunt the peak of the pattern on 2m rather than the null, as it is less affected by reflections
and common mode current, and tends to be much more stable over frequency.  But that requires
a good attenuator (preferably the "offset" or "active" type) and a receiver that can show small
variations in signal strength.  Fortunately the S-meter in many FM rigs has only about a 12dB
range, which works well enough with "eyeball averaging".  For serious work, the VK3YNG "sniffer"
has an audio S-meter that allows me to detect a 1/4dB difference by the pitch of the tone,
which makes bearings off the peak much more accurate.

Meanwhile, here is G3ZOI's modified construction of the WB2HOL "tape measure" yagi using
commonly-available British parts:

And here are my modified dimensions if you want to optimize it for gain/forward pattern
rather than the rear null:

Posts: 551


« Reply #6 on: January 29, 2015, 10:11:43 AM »

Its G3ZOI's design i'll be going with,  ive used one in the past, but not built my own before

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