78cm (30.7 in) Diam Loop for 10-15 Meters

mark3A IMG_20131005_115638

 

19May2014.  On the left is the 4th generation antenna – the 4th try at a 10meter loop.  The primary loop is 8 feet of 7/8″  or .88″ 50Ω heliax hardline.  On the right is the 5″ square variable cap is constructed from parts produced by Bob (ve3uk) – moderator of the Mag-Loop Yahoo group.   Tuning is remotely controlled by radio using a standard servo with a 6:1 planetary reduction drive.   The feed loop is a 1/5 c  loop of 1/4″ copper tube.  I use RG6 transmission line.  Mark 4 is a temporary state. I am not satisfied with its 10 meter performance but find it useful on 15 meters.   It does work but it’s amazing what will work (e.g.  a “wet noodle” antenna) with good propagation.  

On 10m it is not my “go to” antenna since the horizontal end-fed on the roof which is my  “standard” consistently outperforms by as much as 12 dB in many but not all QSO situations.  There are situations where the loop has 6 dB gain over the fixed end-fed (EF) on 10m.  Still trying to figure this.  Often to make the QSO the EF will seal the deal.

This loop approaches “go to”  status on 15M however with unity gain or better in many situations vs the EF.

Band Tuning Guide

  1. COAX #3 to switch position #3 on antenna selector
  2. Insure 6V connector is solid and RC is working.
  3. The set screw (2mm  | 5/64 )needs to be torqued down or it will slip.
  4. Orientation :  120°

10 Meters  (  1:1  Rs = 48Ω  Xs = 4Ω )  Bandwidth  169 khz

  1. Move cap to the 10 meter mark  counterclockwise from 15M position reducing capacitance
    1. 28.310 = 63° outside stator edge past maximum cap (or  27° past minimum capacitance)
    2. 28.120 = 61.7°
  2. move feeder to 10 meter position with feeder loop coax connector at the middle of the primary conductor

15 Meters  ( 1:1  Rs = 53Ω  Xs = 0Ω )   Bandwidth  99 khz

  1. Move cap to the 15 meter mark.   (clockwise from 10M position  increasing capacitance)
    1. 21.07 mhz  = 13° outside stator edge, i.e.  past maximum capacitance or (77° past minimum)
  2. move feeder to 15 meter position about two inches above the primary

General Capacitor Tuning Calculations
140 khz per degree  or  .007 deg per khz.
If the servo range after reduction is 5°  the tuning range is about 700 khz.

From measurements taken from the 10 and 15m marks.,  we see a range of 7.24 mhz  in 50° or  .140 mhz per degree …        140 khz per degree,  or .0071 deg per khz
s8.310 to 28.120 mhz = 190 khz or 1.35°  rotation of capacitor to tune.

 

Placement – Corner of Fence away from Everything

This loop is very sensitive to everything in its surroundings, which is unfortunate because of all of the objects that are inherently part of its environment, namely my QTH.   My mag-loop experience leads to the observation that 10m performance is more affected by environment than at lower frequencies.  SWR is the most immediate indicator, followed by bandwidth.  I don’t get close to the minimum computed bandwidths with this antenna, but so far 15 meter performance seems to be more consistent.

Table 1 LOOP CALCULATIONS
Circ 8.00 ft Diam 2.54 ft Conductor 0.88 in      Area 5.09 sq ft
2.44 m               0.78 m                     22.22 mm            0.47 sq m

Distributed Capacitance 6.56 pf
FREQ     WL Eff%        Cap  pf       Q     BW khz     Gain       Vcap    watts    ERP
5.000   60m  05.09  566.67  2584   1.9       -12.9dB    3785  100         5 W
7.188    41m  16.05  270.81   2741   2.6       -7.9dB      4674  100       16 W
14.000 21m  66.35   66.56    1533    9.1       -1.8dB      4879   100       66 W
18.000 16m  82.61   37.67      898    20.0   -0.8dB     4235    100       82 W
19.000 15m  85.17  33.14       787    24.1    -0.7dB    4073    100       85 W
20.000 15m  87.29 29.27      692     28.9    -0.6dB    3918     100       87 W
21.000 14m  89.07 25.94      610     34.4    -0.5dB    3769     100       89 W
22.000 13m  90.56 23.05      539    40.8    -0.4dB    3627     100       90 W
24.000 12m  92.86 18.32      426     56.3    -0.3dB    3367     100        92 W
28.000 10m  95.71 11.72      276   101.3   -0.2dB    2930     100        95 W
28.460 10m  95.94 11.13       263   107.8   -0.2dB    2886     100        95 W

 


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