Directional (Loop) Antenna for Longwave?

[ChrisSmolinski]

I'm looking for ideas for a directional loop antenna for longwave.  This would be an outside antenna, but something small enough that it can still be rotated. Also, it needs to be a non resonant design, or at least have a low enough Q that it covers the entire DGPS band. An amplified antenna would be fine.

I recall many years ago having an indoor loop antenna for LW, it was, as I recall, a magnetic loop with a metal shield, and it plugged into a metal box housing the amplifier. The interface was pretty clever, it was a 1/4" headphone jack, so the signals could pass, and it was able to rotate. Obviously I want something larger than this, and it needs to be outside, unlike back in the 80s, there's too much RFI inside the house today. But perhaps an amplified magnetic loop is one avenue to consider?

My end goal is something that will be automatically rotated back and forth at a very slow rate, perhaps one turn every hour or half hour, meanwhile my SDR is recording the entire DGPS band. I want to try and maximize the total number stations I pick up, and this sounds like one possible way to do it.

[jFarley]

Non-resonant, wideband, and small enough to swing on a rotor.  I can think of a couple of possibilities.

The Wellbrook ALA100 is a favorite among NDB DXers, and available off the shelf.  But then there is the cost.

http://www.wellbrook.uk.com/ALA100M-1

Some have also gotten great results from the LZ1AQ loop at LW.

http://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm

[ChrisSmolinski]

Thanks jFarley, I'm reading through the LZ1AQ page now. It looks like it has some potential. And it's great to see that the amplifier uses common 2N2222's, not some special low noise transistors that went out of production a few years ago and are nearly impossible to find 

[skeezix]

I have both an ALA1530S+ and ALA100LN.

The ALA1530S+ is back out in the shed and use it for LW-SW. It works very well on LW. Recently picked up the ALA100LN and have a ~43' wire loop connected to that. The loop is stapled to the outside of my porch and is in the N-S plane. Comparing to a small active whip I have, it cuts out a lot of noise (which is hardly surprising, but does prove its success). I frequently listen to 540 WXYG on it and its an absolutely solid signal, from a 850W station ~75 miles away.

I would highly recommend either of these. With the ALA100LN, you can customize the size of your loop.

Tuning to 317 kHz now, its a rock solid S6 with its data stream. Being ~15 miles away, it better be. I think its mostly ESE from me, so coming in on the broadside of the loop, so signal is prob not as strong as it can be. And with the antenna stapled to the porch, hard to turn it.

A friend of mine recently suggested to set up a pair of crossed deltas that are phased. The direction can be remotely steered by changing the remote termination (variable resistors) at the far end.

For remotely & automatically changing the variable resistors, perhaps controlled by a stepper motor.

[Kilokat7]

For remotely & automatically changing the variable resistors, perhaps controlled by a stepper motor.

Vactrol's also work well in this application and might be a simpler solution (no moving parts).

[skeezix]

Vactrol's also work well in this application and might be a simpler solution (no moving parts).

YES! I forgot about that and my friend (and others) were talking about that too.

[Kilokat7]

For Vactrols, the VTL5C4 seems to be popular, and I just used a pair of them in a new D-KAZ loop controller that I built.  Unfortunately they are getting hard to find now.

[VE4VE]

I second the ALA-100 loop form Wellbrook. I'll be posting a review shortly again on the equipment reviews, but FYI, Andrew at WB has recently released his latest iteration of this antenna, the ALA-100LN-M. It has a VERY low noise floor VERY good gain figure (+26dB power gain) and can be used with a variety of wire loops from 4-8m or so. For exclusively LW listening, a longer 6-7m wire would probably be best.

I use a 484cm loop and MW is nuts. I have to drop my gain down to sift through the sheer volume of MW DX I can get on this loop - just crazy!  (in a good way)

Tons of LW/LF beacons hit my currently fixed E-W orientation of my loop. I plan to set it up to rotate it this spring, but cold weather over our winters here is not fun to work in and not very coax or wire friendly.

IMO, this antenna is THE best for what you hope to achieve. NF << 1dB, even to 1MHz. VERY low, lower than atmospheric noise, so you'll only be limited by that.

Worth a serious look - the new amp uses 8xFETs in a very low noise push pull and cascode configurations. I have loops #406 and #407. Not sure if that's since his first loop run, or the new iteration, but form what Andy has told me I am one of the earlier recipients of these new loop variants. So I may be in the first 10 users (?).

I'll post specs and such below here for you all here to review:

Hello Darren,
 
Thank for taking the time out for the review.
 
Here are the answer to your questions
 
1.  The new gain spec. is a power gain of 26dB. The lower gain belongs to the older version. I will take a look at the website to make sure its up to date. The new ALA100LN is basically the same as the new FLX1530LN
 
2. The measured Zin is approx. 28 Ohms at 1MHz raising to approx.. 50 Ohms at 30MHz. Your loop z calculation looks okay. The loop circumference shouldn’t exceed 7m for HF, but can be made larger for LF. 
With a 4m loop the Antenna Factor will tend to be flat. This is because the loss due to the loops series inductance is compensated by the increase in radiation resistance with a raising freq. The JFET design ensures a very low noise floor e.g. at 2MHz the ALA100LN would have a 16dB lower noise floor compared to the W6LVP using the same loop size. Also the IMD performance is exceptional and so is the signal handling with a  1dB comp. point of +25dBm.
 
3. Providing that loop circumference doesn’t exceed 0.2 Lambda the pattern will not change i.e. it will still have 2 nulls. The best near field E –field rejection is when the circumference is less 0.1 Lambda. The loop is connected to a transformer with a center tap to ground to ensure max. common mode rejection i.e. E-field.
 
4. The loop will be okay with Li-ion and lead acid gel batteries; the voltage should not exceed 14V. I would recommend a 500mA slow-blow fuse when using high current batteries.
 
5. Others have tried using the amp.s with a Ferrite sleeve. The bottom line was only the ALA1530LNP with a 1m loop. could provide the best broadband performance. This was done in quite location overlooking the sea on the Oregon state highway.
 
Hope this helps
 
73
 
Andrew
 
Loop noise figures  See. measuring noise figure on my website for further info...


f (Hz)        NF (dB)
200k           2.12     5.0     1.76   >8.0   >10
500k    1.45     4.0     1.20   >8.0   >10
1M           0.80     1.45  0.79   8.0     10
2           0.51     1.0     0.51   7.0     4.0
3           0.33     1.0     0.33   7.0     2.0
4           0.27     1.0   0.27   5.5        2.0
5           0.21     1.0   0.27   4.7     n/a
6           0.21   1.0     0.27   4.0     1.0
7           0.21   1.0   0.33      3.0     n/a
8           0.21    1.0    0.33    3.0    n/a
 
In order these are for (left to right):
ALA1530LN/P (new)
ALA1530
ALA100LN (9uH wire loop - the new LN-M version above)
W6LVP
Modified M0AYF


 
 
 

[JCMaxwell]

You may also want to consider one of W6LVP’s options -

https://www.w6lvp.com

Mine does well on LW and I don’t even have it on a rotor.

His experimenter kit may be right up your alley.

[KaySeeks]

And with the antenna stapled to the porch, hard to turn it.

I'm visualising a house jacked up on a platform. Much simpler to rotate in that case.