The small SWL beam (1...30 MHz)

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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

Just for the sake of discussion, let's take the 40m band, now a Yagi-Uda antenna for that band would have two elements with a total length of about 20m and a boom of about 5m, such an antenna installed at 10m from ground will have the following radiation pattern
yagi40.jpg
yagi40.jpg (167.68 KiB) Viewed 1067 times
now let's look at the radiation pattern of the small loop, installed at an height of just 3m on the same band
loop40.jpg
loop40.jpg (167.1 KiB) Viewed 1067 times
if we let aside the gain, which for a receive only antenna isn't the main factor, as field tests of the loop (and tests with the LoG and other antennas) demonstrated, I believe that the very small loop offers some remarkable features, even if compared to a big and high Yagi beam, as an example, look at the lower angle of the beam or at the front to back ratio, the latter is about 9dB for the Yagi and a whooping 29dB for the small loop, while the lower radiation angle is 20° from the horizon for the Yagi and 10° for the loop, if one considers that the Yagi is up at 10m from ground while the loop is just at 3m the thing is pretty remarkable, also considering the fact that to lower the Yagi angle down to 10° we should raise it at 1/2 wave, which, on the 40m band means about 20m from ground

forgot, for the curious ones, the NEC model for the Yagi-Uda antenna is this one

Code: Select all


CM ----------------
CM Yagi-Uda antenna
CM ----------------
CE

' symbols
SY freq=7.100             ' calc frequency
SY wave=(300/freq)        ' wavelength
SY hght=(wave*0.25)       ' height from ground
SY wire=0.0050            ' elements radius
SY lref=(wave*0.495)/2    ' reflector length
SY ldip=(wave*0.473)/2    ' dipole length
SY ldir=(wave*0.440)/2    ' director length
SY sref=(wave*0.114)      ' reflector spacing (from 0.1 to 0.25)
SY sdir=(wave*0.206)      ' director spacing (from 0.1 to 0.5)
SY segm=51                ' segments in model

' antenna geometry
GW  1 segm   -ldip       0  hght   ldip     0 hght wire  ' dipole
GW  2 segm   -lref   -sref  hght   lref -sref hght wire  ' reflector
GW  3 segm   -ldir    sdir  hght   ldir  sdir hght wire  ' director

' terrain (standard)
GE -1
GN  2  0  0  0  13  0.005

' loading (aluminium)
LD  5  0  0  0  37700000

' enable extended kernel for calc
EK

' feedpoint
EX  0  1  (segm/2)  0  1.  0  0

' test frequency
FR  0  0  0  0  freq  0

' end of model
EN

the model is for a 3 elements antenna, willing to test a 2 elements one, just comment out the director line (or the reflector one, if you prefer), notice that the ideal height of the antenna should be about 1/2 wavelength (0.60 being a good pick), I used 1/4 lambda but that can be changed too in the NEC model, just consider that it would mean raising the antenna at around 20m from ground
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

Just to add a bit, when Ollie (13dka) posted the first part of the document regarding the "SULA" (was "SDL" :D), one of the commenters ("Vince", thank you) posted a link to an antenna designed by W2PM, and that raised my curiosity, since not only the concept was the same one, but the antenna was bigger and lower on ground, so I decided to give it a spin by modeling it in NEC

Code: Select all


CM ----------------------------------
CM File: W2PM.nec
CM
CM W2PM Mini Diamond Receiving Flag
CM impedance 800 Ohm (16:1 balun)
CM https://www.ok1rr.com/index.php/antennas/9-the-w2pm-mini-diamond-receiving-flag
CM ----------------------------------
CM
CE

' symbols definition
SY freq=7.100               ' test frequency
SY hght=1                   ' height of bottom corner from ground
SY side=1.5                 ' length of one side
SY diag=(sqr(2)*(side/2))   ' half diagonal
SY wire=0.00125             ' wire radius
SY vres=660                 ' loading resistor value
SY segm=21                  ' number of segment in wires
SY segs=5                   ' short wires segments
SY wfed=1                   ' feedpoint wire
SY sfed=segm                ' feedpoint segment
SY wres=4                   ' wire hosting the resistor
SY sres=1                   ' segment hosting the resistor
SY drop=hght-diag           ' coax drop section length
SY spac=(wire*5)            ' spacing to simulate coax routing
SY dist=diag+spac           ' spacing for coax feedpoint
SY coax=(wire*2)            ' simulated coax radius


' wires geometry
'  ID seg    x0     y0   z0         x1     y1     z1          wire rad
GW  1 segm     0     0  hght      -diag     0   hght+diag       wire
GW  2 segm     0     0  hght       diag     0   hght+diag       wire
GW  3 segm -diag     0  hght+diag     0     0   hght+(diag*2)   wire
GW  4 segm  diag     0  hght+diag     0     0   hght+(diag*2)   wire

' coax feeder "simulation"
GW  20 segs -diag spac hght+diag      0  spac  hght+diag  coax
GW  21 segs     0 spac hght+diag      0  spac  spac       coax


' ground parameters
GE  1
GN  2  0  0  0  13  0.005

' wires loading
LD  7    0     0    0 2.1 wire    ' insulation
LD  5    0     0    0 58000000    ' wire
LD  1 wres  sres sres vres   0    ' resistor

' enable extended kernel for calc
EK

' feedpoint
EX  0  wfed sfed 0 1 0 0

' initial test frequency
FR  0  1  0  0  freq  0

' end
EN

and running the model I got some interesting results, for example, here's the above NEC model on the 40m band

Image


as you can see, the pattern is opposite to the SULA one (toward the resistor) and the backside "null" is missing, this isn't to say that the W2PM is bad, but the choice of routing the coax "inside" the antenna and the low height on ground, make a difference here, on the other hand the gain is -32dB, which is better than the SULA, but then the vertical beam is wider (5° from vertical), which means that the antenna will be sensitive to NVIS signals, now let's look at the SULA, the model is the following

Code: Select all


CM ----------------------------------
CM File: SULA.nec
CM ----------------------------------
CM
CM Small Unidirectional Loop antenna
CM
CM feed using a 9:1 balun transformer
CM place at 3mtr from ground to avoid
CM pattern distortion, optionally add
CM a 20...30dB preamplifier (LNA)
CM
CE

' symbols definition
SY freq=7.100               ' test frequency
SY hght=3                   ' height of bottom corner from ground
SY side=0.762               ' length of one side
SY diag=(sqr(2)*(side/2))   ' half diagonal
SY wire=0.00125             ' wire radius
SY vres=530                 ' loading resistor value
SY segm=13                  ' number of segment in wires
SY segs=5                   ' short wires segments
SY wfed=1                   ' feedpoint wire
SY sfed=segm                ' feedpoint segment
SY wres=4                   ' wire hosting the resistor
SY sres=1                   ' segment hosting the resistor
SY drop=hght-diag           ' coax drop section length
SY spac=(wire*5)            ' spacing to simulate coax routing
SY dist=diag+spac           ' spacing for coax feedpoint
SY coax=(wire*2)            ' simulated coax radius


' wires geometry
'  ID seg    x0     y0   z0         x1     y1     z1          wire rad
GW  1 segm     0     0  hght      -diag     0   hght+diag       wire
GW  2 segm     0     0  hght       diag     0   hght+diag       wire
GW  3 segm -diag     0  hght+diag     0     0   hght+(diag*2)   wire
GW  4 segm  diag     0  hght+diag     0     0   hght+(diag*2)   wire

' coax feeder "simulation"
GW 20 segs -dist     0  hght+diag -dist     0   drop            coax
GW 21 segs -dist     0       drop     0     0   drop            coax
GW 22 segs     0     0       drop     0     0   spac            coax

' ground parameters
GE  1
GN  2  0  0  0  13  0.005

' wires loading
LD  7    0     0    0 2.1 wire    ' insulation
LD  5    0     0    0 58000000    ' wire
LD  1 wres  sres sres vres   0    ' resistor

' enable extended kernel for calc
EK

' feedpoint
EX  0  wfed sfed 0 1 0 0

' initial test frequency
FR  0  1  0  0  freq  0

' end
EN

the antenna is smaller, since the SULA has a 76cm side, wherease the W2PM has a 1.5m one, also the SULA is placed higher since it's at 3m while the W2PM is just at 1m from ground (more on these later), but the SULA simulation results on 40m were the following

Image

and I think that it's easy to see that, while the SULA shows lower gain (-42dB) it shows a deeper "null" at the back and that its vertical lobe is narrower, with a 15° angle from vertical, which makes it less subject to interference from NVIS signals, this, coupled with the backside "null" will help attenuating undesired signals (and noise)

Now, I ran a number of tests and frequency sweeps and the results of those are pretty clear to me, but I'm not going to post all the results, if you want, pick the two NEC models and run them yourself; by the way, it would be interesting if some of the designers or users of the W2PM could jump in here and discuss the thing, I believe that such a discussion may end with interesting results
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

just to close the loop :D

https://swling.com/blog/2022/08/introdu ... can-build/

so we have a straight link :)
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4nradio
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Re: The small SWL beam (1...30 MHz)

Post by 4nradio »

Hi Andrew & 13dka,

I can't thank you enough for your detailed instructions, NEC modeling results, and observations on this antenna! You've put a lot of effort into documentation and I hope a lot of radio hobbyists give this project a try.

I've used Flags and other terminated loops for years during DXpeditions for SW and MW at the Washington and Oregon coasts, with great results. I've tried Flags a couple of times at home too, but have struggled with noise pickup here.

I'm sure I'll try building the SULA, and will experiment with it on a antenna rotor at home before trekking out on a DXpedition. I have a couple of questions though:

1. I might try using a Wellbrook FLG100LN module as an amp, and this unit has a recommended termination of 900-1100 ohms. Since the SULA is designed for 530 ohms, do you think it would work OK if I added a 2:1 turns ratio transformer to the Wellbrook's loop input terminals to step down the value? (i.e., halve the requirement from 900-1100 to 450-550 ohms. I would then use the specified 530 resistance).

If this isn't feasible I will try a 50 ohms preamp following a 9:1 transformer, per the SULA design. I have some high efficiency MNX binocular cores recommended years ago by John DeVoldere ON4UN, who authored the classic "Low Band DXing" books.

2. Would a larger loop for the SULA potentially increase gain on the lower end of medium wave?

73, Guy
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

4nradio wrote: Mon Aug 22, 2022 9:37 pm Hi Andrew & 13dka,

I can't thank you enough for your detailed instructions, NEC modeling results, and observations on this antenna! You've put a lot of effort into documentation and I hope a lot of radio hobbyists give this project a try.

I've used Flags and other terminated loops for years during DXpeditions for SW and MW at the Washington and Oregon coasts, with great results. I've tried Flags a couple of times at home too, but have struggled with noise pickup here.

I'm sure I'll try building the SULA, and will experiment with it on a antenna rotor at home before trekking out on a DXpedition.
Hi there, Guy, welcome aboard and thanks for your kind words, and yes, we too hope that some people will decide to give the SULA a try and possibly report their experience since such a thing could help improving this small rx antenna, and given your experience with such antennas, if you'll go on and give the SULA a spin, your feedback will be really valuable; just a note, a rotor is ok, but please use a non conductive mast/pole for the SULA since a metal one will influence the pattern, if forced to use a metal pole try keeping it at least 1m (about the length of the "diamond" diagonal) below the antenna bottom corner to reduce such influence (in this case the vertical support for the loop should be prolonged to give the needed spacing)
I have a couple of questions though:

1. I might try using a Wellbrook FLG100LN module as an amp, and this unit has a recommended termination of 900-1100 ohms. Since the SULA is designed for 530 ohms, do you think it would work OK if I added a 2:1 turns ratio transformer to the Wellbrook's loop input terminals to step down the value? (i.e., halve the requirement from 900-1100 to 450-550 ohms. I would then use the specified 530 resistance).

If this isn't feasible I will try a 50 ohms preamp following a 9:1 transformer, per the SULA design. I have some high efficiency MNX binocular cores recommended years ago by John DeVoldere ON4UN, who authored the classic "Low Band DXing" books.

2. Would a larger loop for the SULA potentially increase gain on the lower end of medium wave?
Let me start by saying that due to personal issues I can't run field tests at the moment, so my replies below are based on some NEC simulations, I hope that Ollie (13dka) will chime in and report his field data, all I can say is that till now, the results of NEC modeling the SULA reflected what Ollie observed during field tests, so I'm confident that what follows reflects the antenna behavior

Let's start from the FLG100N, that preamp has an embedded 16:1 impedance transformer at its input, so it claims for a higher resistor value to match the impedance, now, the image below shows what happens when we change the SULA resistor from 530 Ohm to 1 KOhm (I also tried 900 but the result won't change)
sula_res.png
sula_res.png (189.13 KiB) Viewed 779 times
as you can see, the antenna pattern changes, the nice backside "null" disappears and the horizontal lobe widens to 180° so, sincerely I'd avoid such an approach; also, placing a 2:1 (ok 1:2 if we're going to raise the impedance) between the loop and the FLG100N preamp would mean having two transformers in series (our one and the one embedded inside the preamp) and, sincerely, I'm not sure about the effects such a thing could have (although I've some "suspects" and sincerely I don't think the setup will work well), that being said, you asked about size, now, let me start by showing you what happens when we change the SULA size
sula_siz.png
sula_siz.png (186.97 KiB) Viewed 779 times
the above shows the standard (76cm side) SULA pattern and the pattern we obtain by raising the side to 1.5m, while the gain changes from -42dBi to -31dBi, the null starts disappearing, also, if we then try moving from 7MHz to (say) 28MHz the effect of the longer side (larger loop) will be the following
sula_10m.png
sula_10m.png (190.91 KiB) Viewed 779 times
now, if one is only interested in the lower frequencies (say 7MHz and below), increasing the loop size may offer better gain, but if one is seeking for an "all rounder" receive antenna, which is small enough to be easily carried around or used even in case of restrictions (HOA and the like) and which doesn't need to be mounted high, then I believe that the current SULA design should fit pretty well, then for sure it may be possible to further optimize and improve it, but as is now it isn't bad, I think :D and, just as a note, here's the pattern of the SULA at 430MHz
sula_uhf.png
sula_uhf.png (95.83 KiB) Viewed 779 times
as you can see, the lobe is reversed, that is points toward the resistor, but the takeoff angle and unidirectional lobe are pretty good, plus at those frequencies the antenna shows positive gain, so one may just omit the preamp :)

My suggestion, if you can/want, is to try the SULA design "as is", since you have some binocular cores, you may wind your own transformer (but if you want the full coverage, you'll need a transformer capable of working up, to at least 30MHz) and, if you want, you may replace the preamp with a different one but try keeping the antenna design as close as the "standard" one as possible, at least for a start

The above being said, if you'll need further informations or want to further discuss the topic we're here and interested in discussing it and possibly finding ideas to improve it, other than that, the NEC model for the antenna is available here, to use it one just needs to fetch a copy of the free 4NEC2 modeler, load the model and then play with the parameters as desired and if you feel versed, I invite you to try !

[edit]

forgot, as for performance on MW, we already tried a number of changes to improve the SULA performance "down" there, but aside from making it VERY larger, increasing its side has little effect and then, by the way, it will totally alter the pattern at higher frequencies so, while performance at LW/MW may be improved, the SULA would become just another "low frequencies loop" while, as it is now, it's a "general coverage" directional antenna which, I believe, may be better than "just another LF loop", also, I think that some people will miss this and just think at the SULA as a "just another 160m antenna" ... but it isn't that kind of thing, as it demonstrated :D
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4nradio
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Re: The small SWL beam (1...30 MHz)

Post by 4nradio »

Thanks so much for the detailed reply! Although my interests are lower HF and MW, I see the wisdom of keeping this antenna at the design parameters at least to begin.

So, when I make this I'll keep as close to the "textbook" as possible including height above ground. I've never used conductive masts for antennas so I won't have a problem with that. The best designed preamp I have is a Wellbrook 50 ohms input/output module that uses (I think) a Norton negative-feedback design. The gain is 15-18 dB if I remember right. I'll skip the FLG100LN and save it for a normal Flag or DKAZ antenna at the coast.
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

4nradio wrote: Tue Aug 23, 2022 9:43 pm Thanks so much for the detailed reply! Although my interests are lower HF and MW, I see the wisdom of keeping this antenna at the design parameters at least to begin.
Well, the SULA will get down to the upper portion of MW but its "playground" starts around 1MHz and changing its parameters (size, resistor, height...) shown no significant improvement down to MW/LW aside a bit more gain, but the antenna pattern changes a lot and not for better, so I think that starting with the SULA design "as is" should be a good idea, then nothing forbids one from trying modifications, in such a case, my humble suggestion is to start by simulating the modification in NEC (or MMANA-GAL if one prefers it) and then if it looks promising, trying it on the real antenna
I've never used conductive masts for antennas so I won't have a problem with that. The best designed preamp I have is a Wellbrook 50 ohms input/output module that uses (I think) a Norton negative-feedback design. The gain is 15-18 dB if I remember right. I'll skip the FLG100LN and save it for a normal Flag or DKAZ antenna at the coast.
As for the mast, that's fine, see a conductive mast may/will affect the pattern so if you can use a non-conductive one, it will be a good idea; as a side note, running simulations (and 13dka real world tests), the lower height at which the SULA will work ok (even with some slight decrease in performance) is 1.80m (about 6ft) but sincerely, if possible, I'd try keeping it at 3m (about 10ft)

Regarding the Norton preamplifier, it's interesting that you mention it, see while developing the SULA I had some mailing with people which "knows something" about loop antennas and Martin (G8JNJ) suggested to use a Norton preamplifier with the SULA, we didn't go that way simply becase we didn't find a relatively cheap and widely available model, so we stuck with the NooElec LANA HF, but if you have a Norton that would be an interesting experiment (although the gain curves of the SULA call for a 20dB preamp imHo), so please, if/when you'll try the SULA with the Norton, report back !
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4nradio
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Re: The small SWL beam (1...30 MHz)

Post by 4nradio »

The places I DX from a few times a year have significant sea gain (coastal salt water) effect, so I suspect the SULA will perform well there even on the lower end of medium wave.

The 50 ohm preamps I currently have on hand are as follows. The Wellbrook module is an old one that I repackaged in an aluminum enclosure years ago, and added a label with the specs. It is 10-15 dB gain, not the 18 dB I mentioned earlier. Perhaps this one will not give enough gain?

The other amp is also an oldie from Advanced Receiver Research (USA). It is rated 20 dB gain, at the low end of what's recommended for the SULA. Both amps have a high 1-dB compression point so they work well in higher RF environments.

Unfortunately these amps are not set up for DC power via the coax (bias-T / voltage injector), unlike the other Wellbrook units I have (FLG100LN for instance). I would have to power them in the shack with 12 VDC, not ideal in my opinion. Locating the preamp at the mast would be better (I'm referring to home usage, where my coax run is about 100 feet long).
Attachments
Wellbrook Amp.jpg
Wellbrook Amp.jpg (167.67 KiB) Viewed 713 times
ARR Amp.jpg
ARR Amp.jpg (156.01 KiB) Viewed 713 times
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Andrew (grayhat)
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Re: The small SWL beam (1...30 MHz)

Post by Andrew (grayhat) »

4nradio wrote: Thu Aug 25, 2022 1:09 am The places I DX from a few times a year have significant sea gain (coastal salt water) effect, so I suspect the SULA will perform well there even on the lower end of medium wave.
Some notes from "13dka" seem to indicate the same, given that his listening post is near the North Sea shore, so your will be another interesting experiment, please again, report back after your SULA tests
The 50 ohm preamps I currently have on hand are as follows. The Wellbrook module is an old one that I repackaged in an aluminum enclosure years ago, and added a label with the specs. It is 10-15 dB gain, not the 18 dB I mentioned earlier. Perhaps this one will not give enough gain?

The other amp is also an oldie from Advanced Receiver Research (USA). It is rated 20 dB gain, at the low end of what's recommended for the SULA. Both amps have a high 1-dB compression point so they work well in higher RF environments.
Well, the SULA curves, over the 0.1 ... 30 MHz range are the following
sula_curves.png
sula_curves.png (217.22 KiB) Viewed 701 times
now, if you look at the gain curve (right click the image and open in a new tab to see it full size), you'll see that a 20dB preamp will fit pretty well, not that the gain here is really important, since we want to set the SNR, but a bit of boost is useful to have enough signal for the receiver; that being said, trying different preamps/gains would be another interesting experiment
Unfortunately these amps are not set up for DC power via the coax (bias-T / voltage injector), unlike the other Wellbrook units I have (FLG100LN for instance). I would have to power them in the shack with 12 VDC, not ideal in my opinion. Locating the preamp at the mast would be better (I'm referring to home usage, where my coax run is about 100 feet long).
As for Bias-T, any device can be powered using a pair of Bias-T units :D ! There's no need for an embedded one, think at it for a split second, got it ?

If not, here's how it works
bias-tee.png
bias-tee.png (19.48 KiB) Viewed 706 times
the upper Bias-T unit is at station side, the "RF" side of the Bias-T goes to the coax feeding the receiver and the DC connector to the power supply, the "RF+DC" side of the unit is then connected to the coax going to the antenna, at the other end of such coax (antenna side) we have a second Bias-T unit connected the OPPOSITE way, that is, the "RF+DC" is connected to the coax coming from the station, the "RF" side goes to the preamplifier, while the DC connector goes to the preamplifier power input

Easy, isn't it :D ? The only "problem" is finding an appropriate Bias-T unit, by "appropriate" I mean a Bias-T which will work over the desired frequency range, because see, a Bias-T unit is little more than an inductor and a capacitor as shown here

Image

now those two components determinate the min and max frequency range, outside that the signals will be attenuated, so it's important that the selected Bias-T unit covers the desired range, then there are a number of such units available, ranging from the cheap ones like this "unnamed" unit to the higher range ones like those sold by "minicircuits" (note: I have no relations with them or with NooElec) and others (as an example, this one may fit), the difference here is the insertion loss and component quality, but in any case the use is the one I described above, one unit injects the DC, another one extracts it :D

[edit]

Just to be more clear, let's look at the SULA gain curve between 0.5 and 1.6 MHz (the MW band)
sula_mw.png
sula_mw.png (48.3 KiB) Viewed 697 times
as we can see, the gain at 0.5MHz is -85dBi while at 1.6MHz it raises to -66dBi, this means that, using a 20dB preamp the gain will range from -65dB to -46dB which should be enough to allow a decent receiver to pick up signals on the MW band, on the other hand, the gain at 30MHz is -19dBi so those 20dB from the preamp aren't too high to worsen the SNR :)
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4nradio
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Re: The small SWL beam (1...30 MHz)

Post by 4nradio »

Thanks for the reminder of back-to-back Bias-T boxes! I had forgotten that I can use them this way. I have an extra identical one to the Bias-T that's powering my ALA1530LN right now (not the Wellbrook supplied unit, but a ham-made pair of Bias-T's). These work well from MW on up.

What do you think of the idea of a circular SULA, with a circumference identical the square version you've modeled, and that 13dka has built? I have some construction ideas for a circular loop of plastic tubing, with the wiring & term. resistors inside, and the feed point transformer also in its proper location.
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