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Topband: Magnetic Slot Antenna

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Subject: Topband: Magnetic Slot Antenna
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Date: Thu, 11 Jul 2002 23:13:04 EDT
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).

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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