[IRCA] TECH: Improved LF for Wellbrook ALA1530LNP

[Steve Ratzlaff <ratzlaffsteve@xxxxxxxxx>]

I recently got a new Wellbrook ALA1530LNP antenna with its 3 foot 
diameter loop and Interface with integral +9 dB amplifier. The headend 
loop amp has been improved for lower noise plus it's now supplied as a 
separate box from the loop assembly itself. Greatly increased LF 
sensitivity can be obtained by using the headend amp with a larger loop. 
Loop sensitivity is directly proportional to loop diameter (area) and to 
frequency. Thus loop sensitivity suffers as frequency decreases, and at 
LF is significantly less than at HF, for a given loop diameter. Previous 
untuned loop experiments that I made from several years ago indicated 
for decent LF sensitivity the loop should be at least 6 feet in 
diameter; my tests went to 10 foot diameter loops, with even better 
sensitivity. Currently an eBay seller in Nebraska offers 30 feet of 1/2 
inch aluminum flexible fuel line for $30 with shipping. Many other 
sellers offer 25 foot lengths (about 8 foot diameter loop). I went with 
the 30 foot circumference for an approx. 9.5 foot diameter loop.

Antenna comparison results

I have a 900 foot unterminated E/W longwire and my weakest 
consistently-heard daytime NDB is 251 SV in Silver City NM, 185 miles. 
Rarely, 251 AM Amarillo, TX at 580 miles can also be heard but 
conditions need to be very good. 242 EL El Paso TX at 282 miles is also 
consistently heard but it's a much stronger signal than SV so I don't 
use it for comparison tests. The new 9.5' dia. loop oriented E/W hears 
the same weakest daytime NDBs that the longwire hears, in fact at 
slightly greater volume due to the Interface amp, so I consider the new 
loop to be a resounding success. Thanks, Wellbrook! Reception in MW and 
HF is also very good so it doesn't appear even HF reception suffers any, 
at least that I can tell. So the much smaller 9.5' dia. loop appears to 
be roughly equivalent to the 900' longwire at least at LF.

73,

Steve AA7U

near Sahuarita, AZ

Materials

3 ten foot lengths of 1 1/4" Schedule 40 PVC Pipe

1 PVC cross, 1 1/4"

Can of PVC cement

30 foot roll of 1/2" flexible aluminum fuel line. The eBay seller is 
"speedway_motors", search for "1/2 aluminum 30 long fuel line tubing"

7/16" wood dowel

two 1" 4-40 stainless steel screws with flat washers, lock washers and nuts

small gauge iron wire for securing loop to PVC frame

vertical support for the mast-- of your choosing--metal tee-post; sturdy 
wooden post,  existing support, etc.

Construction

Sand two inches of the dowel down so it's a very snug fit in the tubing 
ends--cut in half and use each half in each end (tap in place with 
hammer). Drill 1/8" hole through tubing and dowel, 1/2" from end.

Glue the PVC cross to one full ten foot length of PVC pipe--this will be 
the mast and loop frame.

Measure 4'8" on the remaining two lengths, cut off and glue to the PVC 
cross for the three top loop frame arms. (You'll need to measure and cut 
a second time on one piece.) Drill 1/8" hole in the top 3 arms for 
securing the loop to the frame--make sure to drill perpendicularly if 
it's not obvious to you how the holes should go!

Lay the loop frame flat on the ground. Gently unroll the tubing. Measure 
from one end to the center, 15 feet and wrap black electrical tape for 
at least several layers to give a cushion when tying the loop to the 
frame. Measure 7.5' from the ends, wrap tape at those points, for the 
three loop mounting points to the loop frame arms.

Using a length of the small gauge iron wire, tie the top loop in place 
(center of the tubing). Then gently bend the tubing for a symmetrical 
bend on each side and tie each wrapped side to the two horizontal loop 
frame arms.

Gently bend the tubing ends so they meet at the bottom vertical section 
of the loop frame. You can either leave them loose or tape them to the 
mast. Make sure you do not use anything metallic as you don't want to 
short out the loop! If appearance is a factor you could paint the loop 
and frame with a blue or grey or brown, etc. color. That would stop the 
loop tubing from reflecting and also protect the loop PVC frame from 
eventual UV damage.

Stand the loop up noting it's fairly light weight. Attach it to your 
vertical mast support. Attach the headend amp two wires to each end of 
the loop tubing with the 4-40 screws and tighten very securely--the 
inner dowel prevents the tubing from collapsing so you can tighten very 
tightly. You want as low resistance from tubing to the amp wires as 
possible. Secure the amp to the mast frame with ty-wraps or however you 
wish. Attach your coax cable and your loop is ready for use.


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