By Jock Elliott, KB2GOM
It was research on preselectors that led me to this: Improving HF Reception – The RadioReference Wiki
In that section of the RadioReference Wiki, written by Mike, KA3JJZ, I found the following:
Another way an active preselector could be used is to use it to load a very short dipole – say not more than 1 meter (roughly 3 foot) for each leg. A number of years ago, a company called Datong marketed such an antenna (with a preamp right at the antenna feedpoint) that was popular in Europe and to some lesser extent in the Americas because it’s easy to hide the antenna.
In the magic of my mind, I could picture using the short dipole strung across two windows — or a large window — in an apartment or condo (or anyplace with antenna restrictions) and connected to an MFJ 1020C. I had not yet done the experiment, but I had tested the 1020C and found it to be a worthy piece of gear. So I thought the short dipole/1020C experiment might be worth a shot.
The heart of the short dipole, the LDG 9:1 unun.
So I cut two yard-long lengths of wire and attached them to a 9:1 unun. I created loops at the outer end of each leg of the dipole and hung the whole assembly from a curtain rod covering two windows. A coax links the short dipole to the MFJ 1020C, and a jumper connects the output of the 1020C to my Satellit 800 receiver.
The completed dipole. Not fancy, but it works.
Bottom line: it works. The vast majority of the time, even when the 1020C preselector is in bypass mode, the short horizontal dipole/1020C combo delivers a better signal-to-noise than the Satellit’s vertical telescopic antenna. (In rare cases, they are equal.) And when the preselector amplification circuits are activated and properly tuned, the signal is usually improved, often significantly. The Big Trick is to use the preselector to peak the noise at the frequency you want to hears and then tune slightly to the side of best listening.
Obviously that would be cumbersome for band-scanning, but you could band-scan in bypass mode and then tweak the “hits” with the amplification circuitry. In all, if you are living in an antenna-challenged situation, the short dipole/1020C combo just might make your shortwave listening better.
Final thought: Mike, who wrote the section of Radio Reference wiki that inspired this experiment, said:
You do have to watch your gain otherwise you will get a lot more noise than signal. I did my experiment using an old Palomar preselector. You can also try using a YouLoop as the antenna – it should, in theory, work even better than just a simple whip.
One thing that you could mention in your article is that there is an advantage to having a small dipole like that as the receiving element. Not only is it fairly easy to hide, it can be moved around to find a somewhat quieter location. However the coax should be kept as short as you can, otherwise there is a chance that common mode noise would become an issue – particularly if it runs near computers or other RFI sources
Remember that even a 1 or 2 S unit improvement might make the difference between hearing a signal and not hearing it at all. All we are doing here is trying to improve the signal/noise ratio coming into the unit. That little vertical whip on the 1020c is not likely to be the best choice, and that’s what I am trying to improve upon.
– Mike KA3JJZ
Thanks, Mike!
Hi Jock and readers,
I’m new to SWL. I’m also reading and re-reading your articles on antennas, especially the ones on your Grundig 800 and Tecsun 880. I like the idea of experimenting and learning new skills like soldering. I have no theory other than having fun.
I’m thinking of a fly-weight antenna with 2 Sangean ANT-60 reel antennas and a monaural Y-connector as the most important parts. This one’s for indoors. With necessary modifications, this could be done for camping. Any advice?
I have 2 radios with several kinds of inputs. I believe baluns or ununs will help out. Are these necessary? Maybe not, but I want to learn by doing.
A month ago I knew things like BNCs, alligator clips, etc. existed but was not in any way grounded in continuity and experience.
Any caveats? If I were to take the 2 reel antenna idea outdoors how would I do so safely? How would I ground my fly-weight antenna?
Sincerely,
David
David, your question is probably the same from other people which is just starting to explore the fascinating world of radio waves; this “space” it’s quite limited and won’t allow to expand all the points you exposed, so… may I humbly invite you to join the SWLing forums which Thomas kindly activated ?
In such a case, just point your browser to https://swling.net/ create your account (nothing special needed) and re-post your question on one of the forum sections (the “Questions & Advice” one seems appropriate) and we (me, Jock and other people 😀 !) will be heppy to try helping you
The sound quality in DRM is determined by the broadcaster XHE AAC compression used produces excellent stereo sound even at very low bit rates, making it suitable for HF broadcasting. I often listen to the BBC from 4000 km away in the opposite direction to the main signal direction.
Digital reception uses error correction, so when it is incapable of correcting more errors than 1 error in 10,000 bits, the receiver mutes. This is when AM is intelligible. So the challenge has changed from you trying to work out what is being transmitted to trying to get the poorer signal to be good enough to be heard continuously. Text and images are often repeatedly retransmitted the receiver will only update the display when it knows it is error free.
By 4000km, are you in NA? I would love to see more about DRM on here. So far, I feel like a) I’d never hear it in Canada, b) almost no radios support it, and c) no one is investing in putting in new digital transmitters. I’d love to be proven wrong on any of them.
Hi Joc,
I love your indoor experiment posts. They are very helpful as I too am an indoor antenna person.
May I suggest trying the MFJ 16010, it is a random wire tuner and it works. I get a moderate
to very good signal boost from it on SW & MW. I live in a big city and operate portable radios from within my apartment.
Cheers!
Ward Elliott
Ward,
Thanks. I’ll look into it.
Cheers, Jock
The MFJ 16010 is passive, while your MFJ 1020C is active, but ultimately they are fulfilling a similar purpose in this role: improving the mismatch between your antenna and receiver. 😉
The tuner is using a l-match circuit, while your preamp is using a RF amplifier circuit. The tuner should have superior SNR as a passive device when the resulting signal level is enough to sufficiently drive the receiver, but we are talking about a noisy indoor antenna with noisy HF and MW bands anyway, so a couple dB of active preamp noise is unlikely a significant concern either.
On a personal note, I have used a MFJ 16010 to improve the match between a YouLoop and a portable receiver with good results. Skim down a few posts here:
https://www.hfunderground.com/board/index.php?topic=71326.0
Hi there, Jock, just a suggestion
try replacing the two arms of your dipole with two runs of 3 conductors wire, connect the three wires of each arm in series and then, connect them to the unun (a balun would serve you better), you’ll still have the same physical size, but three time the wire “on the air”
see this for details
https://swling.com/blog/2020/12/how-to-build-a-simple-linear-loaded-dipole-for-low-noise-shortwave-radio-listening/
HTH
Andrew,
As always, thanks. I will have to check that out.
Cheers, Jock
Andrew’s idea seems to work. I have adapted it for my shortened Loop-On-Ground antenna (only 9 feet in diameter using 3-wire rotor cable) and deployed it successfully at the recent ham radio “Field Day” with a portable receiver and Cross Country Wireless preselector.
Another thing to try is to try mounting the dipole wires outside the apartment somehow, just like Andrew’s photos show. The reduction in noise might be worth the trouble.
Tom
I’ve only tested it with dipoles, but I’m planning to try it with a relatively small triangular “skyloop” (RX only), as usual, I’ve run some NEC simulations and they look promising, basically it’s a linear loaded dipole with the two arms ends connected with a single wire, forming a triangle, such a wire is broken at the middle and the two ends are connected to a 780 Ohm (2×390 in series) resistor, the latter flattens the SWR curve from a few MHz up to above 50 MHz, the whole thing is fed using a 9:1 balun, again, didn’t build it yet, but judging from NEC simulations it looks promising
TomL,
Yes, I am planning to give Andrew’s idea a try.
Cheers, Jock
The described short dipole is balanced and already low impedance at HF. Admittedly the actual SNR is unlikely to change much, but an 1:1 balun might fair better for signal transfer levels. YMMV.
Robert,
The sad truth is that I don’t have enough technical knowledge to agree or disagree. I used the 9:1 unun because that’s what I had on hand to construct a dipole.
Can you quantify the improvement in signal transfer with a 1:1 balun?
Cheers, Jock
You can take a look at the simple impedance using a dipole calculator.
https://www.rfwireless-world.com/calculators/Dipole-Antenna-Calculator.html
For example, a 2-meter-long center-fed dipole would have feeedpoint radiation resistance of under 1 ohm at 5MHz. Now try to divide that already incredibly low resistance by 9 with a 9:1 balun. 😉
Even without looking at the complex impedance, the simple impedance numbers show you have a huge mismatch between the antenna, feedline, and receiver when using an very short dipole.
The mismatch becomes worse as frequency drops, though admittedly, none of the above might even matter depending upon your preamp gain, receiver gain, noise floor, etc. Lots of variables.
A few passes of your feedline through a mix 31 or 44 ferrite near the antenna feedpoint would be the “better” design option IMO, but again, it might not do much for your particular situation considering the numerous variables involved.
Alternatively a cheap preamp at the feedpoint would turn your short dipole into an active antenna, thus mitigating mismatch losses, but likewise potentially worsening SNR depending upon your local noise floor, preamp noise factor, etc.
BTW, if you want to get into the more technical aspects of antenna design and measurement, a NanoVNA can be a good hobbyist tool to have IMO. Just be sure to order from a reputable dealer versus a generic eBay clone.
https://nanovna.com/?page_id=21
https://nanovna.com/?page_id=121
I have a H4 model for the larger 4″ screen. Ordered from R&L. Works nicely for my basic antenna measurements and tuning purposes.
We should be careful applying worry about mismatches from ham radio to shortwave listening. In ham radio, we care about how efficient power is fed to the antenna, but when SWL, as long as the signal level is sufficient to the radio to be above the radio’s noise floor, improving match is unlikely something to worry much about.
That said, the match here is *so bad* I wonder if most of the signal the radio is getting is going to be coming from the coax (common mode on the coax and the element of the dipole connected to the shield inside the unun, all acting just like one long wire strung across the room), rather than the dipole elements acting as an actual dipole. Even one long wire is better than the shorter whip.
Agreed on all counts. Still I would be curious to see a complex impedance sweep of the described extremely short dipole, and as you noted, whether the feedline is the actual antenna.
Ideally we should be talking about SNR optimization over actual S levels. Many of my best HF and lower frequency catches have been well under S1 signals via my 148′ coaxial “shielded” loop-on-ground; an extraordinarily lossy antenna at HF and lower by design.
As for the impedance, w/o going on and replicating Jock’s design, I quickly wrote a NEC model and ran it, here’s the graph showing the impedance curves resulting from a NEC sweep from 1 to 30 MHz
https://i.postimg.cc/7LCgCbZc/jock.png
https://postimg.cc/sBrBqjS9
as you can see, R is pretty low, and X is always above it; if you’ve NEC at hand and want to try, here’s the model I used
=================================================
CM short dipole
CE
‘ symbols
SY freq=7.100
SY wire=0.00075
SY leng=1.8288
SY arms=leng/2
SY hght=2
SY segm=51
‘ arms structure
GW 1 segm 0 0 hght -arms 0 hght wire
GW 2 segm 0 0 hght arms 0 hght wire
‘ ground parameters
GE 1
GN 2 0 0 0 13 0.005
‘ wires loading
LD 7 0 0 0 2.1 wire
LD 5 0 0 0 58000000
‘ activate extended kernel
EK
‘ feedpoint
EX 0 1 1 0 1.0 0 0
‘ test frequency
FR 0 0 0 0 freq 0
‘ end of model
EN
=================================================
and if you’d like to further discuss the topic, I invite you to join the forums at swling.net 😀
A dipole should be half the wavelength of the signal wavelength. For the American AM band the signal wavelength is between 554 down to 176 metres as the frequency rises. Dipoles which are too short are capacitive, so these tuners contain inductance to balance out the antenna’s capacitance. This will optimise sensitivity, because the antenna is now in resonance. If the antenna was half wavelength long the maximum signal is received without an antenna tuner.
For high frequency bands they are named in wavelength in metres to give the listener an idea of antenna length. The signal wavelength is 299000/frequency in kHz. or 299/frequency (MHz) metres
Mangosman,
I understand that, ideally, dipoles should be cut for specific frequencies, but this is an attempt to create an antenna for general-purpose listening in antenna-challenged situations.
Cheers, Jock
Hi Jock….I am really appreciating your postings! As a ham operator turning towards adding SWLing….and heading soon to a small condo with people and their RFI issues all around me…… I really am looking for a good antenna solution to support my new Airspy HF+ Discovery and ICF-7600GR. Your postings always inspire and educate.
BTW…when I bought the Airspy…I also bought the Airspy sold version of the YouLoop antenna…
Although I could dig thru your various postings for indoor antennas, I am wondering if you could somehow summarize your “favorite” solutions?? Maybe your top 2 or 3 recommendations??
Anyway…many thanks for your contributions…as a newbie they are very valuable to come up the learning curve…
Jim / AC3B
Jim,
Thanks for the kind words.
A lot depends on the particulars of your situation.
I would start with 20 feet of wire, alligator-clipped to the whip antenna on your portable or the antenna connector on your Airspy.
Then maybe the horizontal room loop: https://swling.com/blog/2021/06/jock-designs-a-horizontal-room-loop-to-cope-with-reception-issues/
Then the indoor end-fed: https://swling.com/blog/2022/05/the-satellit-800-the-tecsun-pl-880-and-two-indoor-antennas-an-afternoon-of-experimentation/
Then maybe add a ground: https://swling.com/blog/2022/05/jock-gets-a-good-grounding/
And, of course, the short-dipole/1020C combo shows promise.
A lot depends on what restraints your are operating under.
I hope this helps.
Cheers, Jock
I would avoid clipping anything to the whip. There is usually a small amp somewhere near the base of the whip in the radio, and one static zap and you’re done. Better to use the antenna connector, if it has one, and build a small box with a cheap pot and one or two other things to feed into the connector. If the box takes a static zap, you still have the radio.
Mike
Mike,
That depends on the radio. The CCrane Skywave SSB, for example, comes with an auxiliary wire antenna that is designed to clip to the whip. Obviously, CCrane has thought about the static zap issue.
Cheers, Jock
Jim,
If you get a condo with a large porch, you can put a loop wire antenna outside like I have. A second loop in a different direction can also be setup and both antennas phased into something like the MFJ 1025 or 1026 (use a portable AM radio tuned to around 1100-1200 kHz and listen for the least noise, and then the most noise Direction, and setup the loops in those two directions in order to phase them).
Loops do not need a ground, which is an advantage for apartment setups. My porch happens to be 16 feet x 8 feet x 8 feet high on a second floor, a very rare type of porch. Magnetwire is useful since it cannot be seen from the street. I have a 20 meter ham antenna loop plus a 5 conductor loop in an “X” pattern and it works well enough to be worth the trouble. Just some thoughts for you.
Is this you, Tom?
https://swling.com/blog/2020/05/guest-post-kk5jys-porch-loop-receiving-antenna/
Mike
No that looks like Matt Roberts’ post. My antenna is a slot loop fed from the side for to give it vertical polarization. See Cebik’s page and look at the Elongated Rectangular Loop: http://www.antentop.org/w4rnl.001/scv3.html .
Cheers,
TomL
I really appreciate the comment from Mike about a one or two S-unit improvement in a signal making all the difference. In a world where so many people are used to hearing crystal clear music and HDTV, we SWL’ers can fall prey to what I call the “FM quality” virus – anything that doesn’t sound like a local FM station we might be tempted to pass by. Part of the fun (at least for me) is digging out hard to hear signals, and a single S-unit improvement really can make all the difference! Cheers!
Robert,
I agree. I’m not a hard-core DXer, but I enjoy teasing out a hard-to-hear signal.
Cheers, Jock