Wednesday, 28 August 2019

My 10’ x 20’ Active Loop

courtesy: americanradiohistory.com



After recently installing a new coax run out to the loop and after five years of summer sunlight, the plastic food-storage container used to house the loop’s preamp (Wellbrook ALA100LN ... LF-30MHz) finally bit the dust ... literally.


Last week I replaced the housing with a slightly larger plastic box but I’ll still be faced with the UV bombardment and eventual deterioration of the clear plastic and lid unless I do something different.


I’ve a couple of thoughts but am hoping for some other suggestions from blog readers.

I have thought about spray painting the container, which may (or may not) prevent UV damage, but wonder if paint will stick to the smooth plastic surface? Maybe if scuffed-up a bit it would stick ... if so, the coating could be annually renewed.

Another idea is to cover it with duct tape ... not pretty, but possibly doing the job. Any other thoughts out there?



Part 2 and Part 3 of this blog were originally published when the loop was first built (2014) and describe the antenna details as well as showing the loop in action ... listening on the NDB and broadcast band as well as showing its nulling capability. It’s been a great performer and  I can highly recommend the ALA100LN for a large aperture receiving loop. I’m looking forward to what will hopefully be a good winter of exciting loop DXing.

Part 2 - New LF / MF Loop



Some may recall my blog back in July (Wellbrook Loop Plans) describing a new loop that I had been thinking about as a possible replacement for my 10' shielded loop. I had been doodling various construction ideas using PVC tubing in an effort to keep it as light as possible, without introducing any metal near the loop. On Monday of this week, I dismantled the 10' loop in preparation for my new experimental rectangular loop.
I've had the Wellbrook loop preamp here for a few months, so it was now or never, while the weather was still 'antenna-friendly'. Now I do understand that this goes against one of ham radio's long-standing traditions that dictates all antenna work must wait until the weather can't get any worse. Antennas built in the winter rain always work much better than ones put up in the summer. Hopefully it's not Wouff-Hong or Rettysnitch punishable but the fall DX season is almost upon us and I'm not waiting for the rain!

The new 'loop' is not really loop-shaped but is rectangular (10' x 20') and more like a Flag antenna shape. I considered a Flag but really don't need any back-end nulling capability since I'm mainly interested in listening to the east and to the north.


The main boom section is composed of two sections of 1" PVC thick-wall (Schedule 40) pipe joined at the center and reinforced with a 10' section of 2" x 2" Douglas Fir. In addition, the boom has a truss of 1/4" Dacron to take out any end-loading sag. The vertical end sections are 3/4" Schedule 40 PVC pipe, fastened with a T at the boom end. The center mast is made of 2" Schedule 40 ABS pipe with a long section of 1 1/2" ABS nested inside that telescopes upward to anchor the truss ropes and give some additional rigidity to the mast.


Main boom and mast construction
This type of construction would lend itself well to anyone interested in a rotating Flag antenna for the BCB / LF or 160m. The costs are very low, the structure is lightweight and the materials are readily available at most hardware outlets.


Looping E/W


Although the preamp is completely sealed and weatherproofed, I still decided to mount it inside a container. The container also provided a convenient anchor to terminate the loop end wires (PVC-coated #18 stranded) without putting any tension on the soldered terminals.

Although I have not had much time to listen, and conditions are still in 'recovery' mode from earlier disturbances, initial indications are that everything is performing as well, if not better, than expected. It certainly outperforms my 10' active shielded loop by a large margin. I have yet to do any serious S/N comparisons between it and my primary LF receive antenna, a large inverted L, which must be tuned to resonance for the desired listening range. I believe that the very quiet loop / Wellbrook combination will provide an overall S/N improvement.

I have always believed that smaller loops provide deeper and sharper front-to-side nulls so I was pleasantly surprised to measure (using Perseus) null depths from 25-30db, on various groundwave signals ... more than expected. Skywave signals also deliver sharp deep nulls in the order of 22 - 25db ... again surprising, but I'll take them!

A brief listen while pointing S-E last evening turned up good signals from 1 kW'ers KYHN (1650kHz) in Fort Smith, Arkansas and KKGM (1630kHz) in Fort Worth, Texas. An early morning listen revealed good audio from JOIK (567kHz) Sapporo, Japan and JOAK (595kHz) in Shobu. Down in the ndb band, little 25-watter 'IP' on 210kHz was an all-time new catch from Mobile, Arizona.

There is still much to learn from this new antenna system but the biggest challenge will be keeping it up all winter. I did lose one of my 10' loops after several years, due to wind, when the main (un-reinforced) PVC mast eventually failed from flexing fatigue. I will tie the ends of the new antenna down when the winds get strong to reduce as much mast flexing as possible. I could however, run the risk of violating another long-standing radio adage ..."if your antenna stays up all winter, it's not big enough". I just can't win.

Part 3 - Loop Listen 


As Murphy would have it, and in spite of the low amount of solar activity, LF/MF propagation has been very poor since getting my new 10' x 20' loop in place. The few front-to-side nulling checks that I have done, have produced results varying from around 20db to 30db, depending upon the signal. I suspect the depth of null is also affected by the signal's arrival angle but there is still more to learn. The pattern seems to be very close to that of a typical circular loop...the classic figure-8 pattern illustrated below as shown on the Wellbrook data that came with my ALA100LN preamp.

Courtesy: http://www.wellbrook.uk.com

More typically, the null is around 21-22 db as shown on this test while listening to the ground wave carrier of the 'YZA' NDB (236kHz) located in Ashcroft, B.C., about 150 miles to the NE. As expected, the null is fairly sharp and the front / rear lobe, fairly broad.



One short check at dusk produced nice signals from  CJBC, the French-language station in Toronto. The past few nights it has been very strong but with a strong echo effect. I wonder if there is more than one CBC outlet here (860kHz), such as a low-power repeater, causing the echo.



At the same time, while still fairly light outside, WCCO in Minneapolis had a nice signal just before sunset.



No matter how poor conditions become, it seems that the Hinchinbrook (Alaska) NDB, 'ALJ' (233kHz), is always strong ... looping north.



Courtesy: https://www.google.ca/maps
My apologies for the video quality. If you know of any software available for making full-screen Perseus video captures so that I might improve my technique, please let me know. Presently I am just capturing them on my I-Pad which leaves a lot of room for improvement.

(Since originally posting the above, I have been using the Tiny Take free screen capturing software, to produce better quality catures of my Perseus recordings. It's also probable that my new iPhone would produce a high quality video of my computer screen compared to the older iPad used for these ones, but it's something yet to be tried.)

Hopefully conditions will only get better as the season progresses and I am able to give the loop a good workout ... before it gets too windy!

8 comments:

Anonymous said...

I use black electrical tape, duct tape and white [ looks like wide duct tape ]. Get a couple more years of container life in our harsh UV environment

Steve McDonald said...

Thanks for your input.

Anonymous said...

aww come on Steve, don't be such a cheapskate. Those food containers cost only a few bucks at any wally-mart. Get two years out of it then go buy another one. ;-)

Steve McDonald said...

Good point! What the hell :-)) Tnx!

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Anonymous said...

Steve, you know I'm just kidding with you about the container. :-)
Being serious, you have a very interesting and informative blog... it's a great resource on what's going on below 500kc. 73

Anonymous said...

What about a wooden box which contains the plastic box?
73 de addi / dc0dw

Steve McDonald said...

Yes! A wooden box would do the job and could probably eliminate the plastic one altogether...but as another has suggested, these things are so cheap, do I really need to worry about it as it should last for 5 years anyway :-))