Topband: Magnetic Slot Antenna

[Steve Ireland]

W2SH wrote:
At 11:13 PM 11/07/2002 EDT, you wrote:
>Has anyone on this list ever done one of these, or seriously pondered such
an 
>antenna?  I've looked through most of the list's archives and have not found 
>any references to it.
>
>The description on which I am currently focused appears on page 18 of the 
>ARRL's Antenna Compendium, Volume 4, "The Double Magnetic Slot Antenna for
80 
>Meters" by Lew Gordon, K4VX.  (Earlier references are to K5RP's "Magnetic 
>Radiators--Low Profile Paired Verticals for HF" which appears on page 39 of 
>ARRL's Antenna Compendium, Volume 2, and a pair of articles by Boyer, 
>published in 73 Magazine for August and September 1976).
>

VK6VZ replied:

G'day Charles and all

A couple of years ago, I read the K5RP article in the ARRL Antenna
Compendium Vol. 2, with keen interest.  After discussing this antenna with
my friend Phil VK6APH, our conclusion was the antenna was very interesting,
but could easily be simplified for much easier construction with the same
performance.  Also - it is really a variant of the full-wave loop antenna,
but in rectangular form.

The antenna is, as I think you have surmised, effectively a pair of short
top-loaded verticals, with virtually uniform current distribution.
Virtually no current flows in the long top wires and the presence of a
second loop in the K5RP design merely serves an impedance matching
function, as a second turn on the antenna. The pattern of the antenna, as
you would expect, is oval/broadside, with some small gain (3dB isotropic)
in this direction, with some attenuation off the ends.

This low-aspect or rectangular kind of single-turn full-wave loop was used
very successfully by a 5N2 station on 160m some years ago (5N2EKM?), fed
with 50 ohm coax half-way up one vertical side and his design is featured
in one of the editions of ON4UN's low-band DXing book.  Been looking for
the piece this morning and can't find it, but I know it is there...

Get yourself a copy of the National Contest Journal, January/February 1999
and have a look at the modelling article carried out by LB Cebik W4RNL on
page 16 - 20 to see some more possibilities of this single-turn rectangular
loop antenna. Also, take a look at N3HBX's practical implementation in the
NCJ March/April 2000.

Unfortunately, the dimensions of my half acre QTH are just too small to
support using this antenna, but it is in the folder marked 'really good
ideas to try' when I move to larger place one day.  I think it would be a
real 'killer', particularly when over a decent ground screen.

Vy 73,

Steve, VK6VZ
 










>What I found most intriguing in the K4VX article were the NEC-modeled gain 
>comparisons which he shows in Figure 9.  For a 15-degree take-off angle the 
>worst-to-best gain progression spans a 5-dB range for these six candidates:  
>1/4-wl elevated vertical, 1/4-wl vertical with 120 radials, K5RP's 
>double-wire magnetic slot (DMS), bobtail curtain, single-wire DMS, double 
>wire DMS.  (Useful gain comparisions appear for take-off angles between 5
and 
>25 degrees). 
>
>The bobtail curtain and single-wire DMS each show 4 dB greater gain than the 
>1/4-wl vertical with 120 radials.  On 80 meters that means that the 4-dB
gain 
>advantage goes to either a bobtail curtain with a lateral profile of about 
>270 feet and an above-ground vertical profile of about 67 feet, or a 
>single-wire DMS with a lateral profile of 227 feet and an above-ground 
>vertical profile of 82 feet.  Dropping the latter's above-ground vertical 
>profile to a more reasonable 52 feet would only reduce its gain advantage to 
>about 3.5 dB, and further reducing the vertical profile to a low 32 feet 
>would put the gain advantage at a still-worthwhile 2.8 dB.
>
>All of the foregoing relates to 80 meters.  Contemplating Top Band, and then 
>doing a quick-and-dirty scaling (doubling the dimensions), suggests that
with 
>a lateral profile of 454 feet (possible at this QTH) and a vertical profile 
>of 80 feet (the maximum height achievable here with arboreal supports), a 
>single-wire DMS, facing broadside towards Europe, might give about 2.9dB
gain 
>at a 15-degree take-off angle over a full-sized and fully endowed--radially 
>speaking 1/4-wl vertical.  Moreover, the 120-foot vertical profile of the 
>latter is not possible for me, and, given a self-imposed 80-foot vertical 
>profile maximum, a 20-foot diameter disk hat would then be necessary.
>
>Has any reader "been there and done (or thought about) that"?  Do any other 
>websites or other information sources come to mind?  I decided to ask here 
>before contacting K4VX and K5RP.
>
>Finally, I am personally discomfited by the term "magnetic slot."  Rather, I 
>see the antenna as two greatly shortened vertical radiators operating in 
>phase, and not as a derivation from the slot antenna used for VHF and UHF in 
>1940s aircraft and fabricated by cutting a rectangular hole in the metal 
>fuselage.  Those who remember the "skeleton slot" antenna of the early 1950s 
>may recall that a similar derivation was claimed, erroneously in my
opinion.  
>
>With advance thanks for your comments es 73,
>
>Charles, W2SH
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