NJDTechnologies

Radio: it's not just a hobby, it's a way of life

Current Operating Frequency and Mode

Probably QRT tonight and in the morning due to storms in the area

You can’t work ’em if you can’t hear ’em!

– Posted in: 630 Meter Instructional Topics, 630 Meters

…that’s a mantra that is common in all aspects of amateur radio, kinda like, “you can never have too many antennas”.

It’s becoming very apparent that there will be a large number of interested stations that will show up on 630-meters once the dust settles and the band opens under Part-97 rules.  How do I know?  I get emails and talk to them at conventions and more often than not it comes up as a point of discussion during CW QSO’s on the ham bands.

There is a lot of interest in this band (and 2200-meters, for that matter) and much of that interest is coming from the guy that lives on a small lot.  That is likely going to translate to small antennas which have a high probability of resulting in equally small signals.  Just as with a conventional QRP QSO on HF, it usually becomes the responsibility of the larger station to pull out the weaker signal and turn lemons into lemonade.  That’s OK because I love a challenge!

Now that the very hectic month of June is over and Hamcom and Field Day are memories, I am beginning to look at improvements in my receive system that will hopefully facilitate completing more QSO’s with smaller stations that might be using systems that are the bare minimum.  That will likely be the case for opening night under Part-97 rules and while guys will learn what does and does not work and adjust accordingly, that will take time.

I have used a pretty good assortment of receive antennas at my station over the last 15 years as an active low band operator.  Most work pretty well for 80 and 160-meters and have allowed completion of DXCC and beyond on those bands.  It gets more complicated when one tries to scale those antennas for the spectrum below the AM broadcast band.  All but the longest beverage antennas are short, too short really, for much of a pattern to develop and loops, including K9AY terminated loops and resonant, bi-directional varieties, require a lot of effort to ensure that they have good balance and a large enough aperture to pull weak signals from the noise.

In the last year my favorite receive antenna has been the resonant, bi-directional loop.

S2020001

The loop, which has been detailed elsewhere on this web site, uses a tuned circuit and isolated, single-turn pick up loop nested inside of the main, resonant loop.  Recent improvements have allowed this loop to get even better by using a transformer to match the nearly 0 ohm pick up loop impedance to   the 50 ohm coax AND isolate the coax shield from that pick up loop, which serves to reduce common mode noise on the shield that can often result in a negative impact to the directivity of the loop.  Amazingly, the common mode reduction is absolutely as simple as using the isolation transformer and an isolated ground rod for the coax shield at the antenna.  The cruddy noise is gone.

My one complaint about this loop is that the physical size, about 7-foot across, is relatively small for weak signal work, particularly when band conditions are poor.  Don’t get me wrong – it hears plenty of signals – but I am now thinking about that next layer of signals that I might want to work on CW at or just below the noise floor.

The solution to this problem is to build a larger loop with a larger aperture.  This creates some mechanical issues and given that I am thinking very large, it also means rotation by the Armstrong method.  That’s OK and its important to state that a new loop is not intended to replace the existing loop but to supplement it.

There are a few existing structural approaches out there, like the one used at WG2XKA which is a variation on the design by VE7SL .  Both are good but wood adds a lot of weight and more than likely this loop will sit somewhere in my front yard.  A 10-foot tall loop can be accomplished but fashioning a portable (movable) base that also is stable might be tough.  The next approach is a wider, shorter loop that looks more like a soccer goal (or volleyball net?).  What I like about this approach is that it can be fashioned from PVC, painted brown to fit in with the trees and foliage, and pass for a soccer or lacrosse goal to the unknowing passer by.

While most designs, including the loop that I currently use, have nicely spaced, parallel wires for the inductance portion of the tuned circuit, its my plan with the new loop to use a bundle of wires within the PVC conduit.  No more nice spacing – its not necessary.

So how big am I thinking about?  For me to gain much with the new loop, it will need to have a substantially larger aperture.  Reconnaissance to the local home improvement store suggests that a 3-legged T-base with 5-foot outriggers on each side will allow a 15-20 foot span of thin wall PVC with minimal sag for heights up to 10-feet.  That’s not to say I wont need a guy-rope on windy days but moving the thing around in my front yard and driveway where trees and shrubs already cover the area should be reasonable, particularly since this antenna is designed to be a point-to-point antenna for use with prearranged QSO’s or listening towards population centers and other areas of interest.

Other options that are available are the BOG or Beverage-on-ground which is, as it sounds, a beverage antenna that is laying on the ground.  That is exactly the way that Dr. Harold Beverage had intended it to operator.  The consequences of a BOG versus a traditional beverage are reduced output on the BOG, which typically exhibits a 200-ohm surge impedance, and a potential for improved directivity for shorter lengths of wire on a given band.  But your mileage may vary.

I recently tried a BOG for 630-meters and although it is very short at these frequencies (about 300-foot long physically with increased perceived length due to velocity factor and proximity to ground), it worked to reduce the noise level very nicely, driving a preamp to increase the output.  I lacked solid directivity, which was not surprising given the length and haphazard selection of terminating resistance,  but the decrease of audible noise and slight increase in desired signal seems to make the antenna worth a look.  The caveat for this type of antenna, at least for me, is implementing it where people won’t trip when you don’t own the land that it resides on.  That’s tougher than it looks because people often stray off the beaten path.  This fact also necessitates a temporary nature for the grounding and ground rods.  I’ve gotten pretty good at deploying and retrieving BOG’s and their termination ground rods under the cover of night but its tedious and I don’t like spending more time fooling with the antennas than operating when the band has a nice opening.  I’ve used BOGs on 80 and 160-meters with good success compared to conventional beverages that were also in use at the time.

S21000024:1 Beverage transformer with isolated windings made from junk box parts

Another option is the modified K9AY loop as described by Jay, W1VD.  In this design, the very versatile K9AY loop is scaled up, which results in longer vertical sections and better spacing between the elements.  And Jay, being the detailed craftsman that he is, adds a number of features to his design, including a home brew Vactrol for remotely controlling the termination resistance and allowances to use CAT-5E cable as feed line in lieu of coax.

I have had very good success on 80 and 160-meters with K9AY terminated loops but my experiences with similarly sized loops have been somewhat disappointing on 630-meters.    With the appropriately-sized support tree, I might find myself trying the scaled up version for 630-meters, which would give me a nice cardioid pattern on 630-meters in selected directions.  For a single loop from the central US, an East-West orientation would be appropriate and quite functional.

These are the things that are on my mind at the moment.  Individuals that are building stations from the ground up for MW and LW should give serious consideration to their receive and transmit antennas.  Doing so will certainly increase the level of enjoyment for the operator and increase the likelihood of completing a QSO.  Existing stations and Part-5 operators should be considering receive improvements and many, in fact, are doing just that at this time.  As they say, the only bad receive antenna is no receive antenna.