HF Portable

K1FM Magnetic Loop

July 2020: Check out the NEW loop !

After describing my mini loops (here and here), this time I am going to write about the classic “full size” version that I have been using for a while.

At Dayton the Hamvention of 2018 I took my loop with me and I mounted it on a PVC support that fitted a military style backpack. It made quite a splash! Eventually, someone took a picture that ended up on the August 2018 QST and the 2019 ARRL Calendar front-covers. That’s when I proposed the QST editors an article which described how to make the antenna. QST accepted but took its time, finally publishing the article with the May 2019 edition.

The K1FM Loop consists of a LMR400 radiator (124″) fed by an LMR240 exciter (25″). The radiator loop is terminated by a geared, multi-stator variable capacitor made by Alps (14pF to 165pF, P/N 42-36251-1). The exciter, instead, is constructed to be terminated by a 3way BNC adapter.
The geared capacitor and the loop configuration have been inspired by the AlexLoop while the exciter construction and the selfie stick by Alpha Loop.

The exciter loop takes a bit of patience in order to be manufactured this way. Quite a bit, actually. Of course more classic approaches can be used as long as the overall size remains the same.
Both loops are mounted on the selfie stick using a custom 3D printed bracket that fits on standard camera mounts.

The other critical aspect of this project is obviously the capacitor. It is the capacitor which defines the bands you can use, the resolution you get (how “touchy” the loop will be…) and – last but not least – how much power the loop will be able to handle. The 15-165pF geared type is some kind of a sweet spot, a very good compromise for a loop this size when used QRP.
Unfortunately Alps no longer makes this capacitor but from time to time they pop-up on Ebay for about $20 each.
You can, of course, replace this capacitor with equivalent ones (similar, non geared capacitors are more common and much easier to find).

The capacitor enclosure is fundamentally just a box that holds the capacitor against the selfie stick, connecting it to the radiator loop.
The box I used is made by uxcell and measures 3.9″ by 2.6″ by 2″. I mount it against the stick using a custom 3D printed ABS bracket.

The list of all necessary materials, from the main loop to the last nylon screw, is here. Overall the cost for a single antenna should be around $60 but the final cost will vary depending on how much you will spend for the capacitor and for the 3D prints.

Performance of magnetic loops this size have been extensively analyzed and reviewed. Mine is no different than the others.
Please do not expect miracles! Ultra-Portable QRP is hard stuff and you will need to rely the 3 P’s: Propagation, Perseverance, Patience.
In a nutshell this loop performs at around the same level of full sized half-wave dipoles or of a vertical antennas. Yet, you are running 5W watts into it therefore plan for the right time, the right band and the right location if you want to work DX on voice.
All in all, I find it a bit lacking on 40 and I don’t use it on that band unless it is inevitable. 20 and 30 are very fine and – when conditions permit – so are 17, 15 and 10.

Other than the loop, I normally carry my FT-817, a WinCamp Headset adapter, a PC Headset, a Palm Radio CW Pico Paddle, a mini tripod and a military backpack with a PVC antenna mount.
I recently added FT8 and wireless headsets, but that’s going to be detailed in other posts.

Finally, I’d like to spend two words on RF safety.
As a licensed Amateur Radio operator you should already be aware of the following but I, for the sake of safety, I’ll repeat what you probably already know.
Loops can be dangerous around people.
Do not exceed power! By doing so you would irreparably scar your capacitor. If you really go heavy, you basically built a soldering machine: sparks will fly and fires might start.
Moreover, even when QRP, this loop will exceed FCC exposure guidelines when used in close proximity. To be on the safe side, nobody should be closer than 2 meters from this loop, regardless of how much power you are using. Finally, do not touch the radiator while transmitting because you could be burnt, electrocuted or both (~800V at 5W, ~4000V at 100W!).

Hacks SDR

HDSDR OSX version 2.76 released

Once again, with Mario’s permission, I bundled the latest revision of HDSDR to be run on OSX. I also created a new rtl_sdr launcher that should be more stable, easier to use and that incorporates the bleeding edge version of librtlsdr.
I tested this on Yosemite, El Capitan and Sierra. My CPU is a quad i7.

Here is a quick vhf/hf demo that I run using my v3 dongle in direct sampling mode:

In order to download it, please refer to this page.

Latest Release: March 2nd 2017

Hacks SDR

HDSDR on OSX the EASY way – SDRplay edition



Today I updated my HDSDR OSX bundle to include support for SDRplay.
Nothing much has changed… If you want to enjoy SDRplay + HDSDR on OSX, all you need to do is the following:

  1. Install the SDRplay drivers (available here)
  2. Download my bundle
  3. Copy ‘HDSR OSX’ and ‘rtl_tcp SDRplay’ on you hard drive
  4. Run the ‘rtl_tcp SDRplay’ server
  5. Run ‘HDSDR OSX’

As you can see, the bundle still includes the original RTL2832U rtl_tcp server. According to the hardware at your disposal, you can use either use one or the other as their functionalities are practically identical.

The SDRplay support was made possible by the hard work of Tony Hoyle so if you want to thank anybody, please thank him.



Hacks SDR

HDSDR on OSX the EASY way (reloaded!)


Catilina broke everything. Sorry


A while ago I published a bundle that easily allowed anyone to run HDSDR and the RTL2832U on OSX. To my surprise the bundle was quite successful, a demonstration that there weren’t (yet) many available alternatives to use the cheapest (and funniest!) SDR radio in the world on a Mac.

After a few years things have improved a bit and some valid alternatives have surfaced: Gqrx and CubicSDR. Even SDR# has been reported to work under Mono but, believe it or not, neither of them seem to work well on my Mac. Actually they don’t even come close to the level of stability and/or the number of features that can be enjoyed when using HDSDR.

Last week I installed a new active RX antenna and I tried to reopen HDSDR in order to test it. Guess what? I realized that it wasn’t compatible anymore with the most recent versions of OSX. It was time to do it all over again… so here I am.

How to install

1) Download the zip archive and expand it
2) Copy rtl_sdr and HDSDR anywhere you like on your Mac

3) Make sure your RTL2832U is connected
4) Run rtl_tcp

6) Click on Start


At this point, if things go as they should, nothing else is required and you should be ready to go. I have successfully tested Yosemite, El Capitan, Sierra and they work great.

January 19th 2016:  This bundle now also supports SDRplay. More details here.
February 12th 2017: HDSDR updated to 2.76. Improved rtl_sdr
March 2nd 2017: Fixed problem in rtl_sdr (thanks Walter KC1DVT)

Final Considerations

This is an experiment. I cannot guarantee that it will work nor that it will not damage your system or your data. Download and use the bundle at your own risk.
I am not the author of the software and I can be helpful only up to a certain point. All I am doing is bundling a bunch of different packages in a way that is easily executable on a Mac.

HF Portable

K1FM Mini Magnetic Loop 2.0

In the effort of improving my ultra-portable HF antenna setup, I first looked into a Capacitively Coupled Magnetic Loop, better known as the Army Loop (or Patterson Loop). The Army loop performed well. It was so portable that I enjoyed using it before, during and after numerous Tennis matches… Here is one example:

The Army loop, however, sometimes presented a little bit of a challenge because in order to tune you need to operate two capacitors, instead of one. Generally speaking, one capacitor affects resonance while the other affects the coupling impedance. The main problem I found is that even when the SWR is very low, such impedance isn’t always what you would expect it to be (around 50 ohm). You can still match it by operating the second capacitor but there really isn’t a way – while portable – to tell how close you are to the perfect match. That’s the main reason why I decided to look for alternatives.

The idea for my next portable loop came from  the Wonder Loop by Wonder Wand (not to be confused with the ‘G4ILO Wonder Loop’, which is unrelated). Wonder Wand’s Wonder Loop looks like an Army loop, but only has one capacitor. After taking a look inside I was intrigued to see a Toroidal transformer. My first reaction was: WFT?!? Then I remembered seeing something similar on the excellent N4SPP page about loops (probably the best page about magnetic loops actually available). Here is what Frank N4SPP has to say about this coupling method:

N4SPP’s coverage of the toroidal transformer coupling solution

The advantages of such design are two:  no need for a coupler loop and only one capacitor. Precisely what I wanted!
At this point you might ask why don’t  I just use the Wonder Loop. The thing is that the Wonder Loop isn’t simply ultra-portable, it is microscopical. It’s radiator is made of a 1 mm thin copper wire… a bit too much, I guess. All I wanted was the same concept as the Wonder Loop but with a radiator of my choice (and at a smaller price!).

Here is the schematic I started working on:


This time I used a smaller Hammond Enclosure, the 1594ASGY. The capacitor, instead, is a dual gang 266 pF by Mike’s Electronic Parts.  Both gangs are engaged in parallel as well as all the trimmer capacitors (trimmers are used to raise the minimum capacitance and also raise the maximum a little bit). By doing so, the loop covers from 40 to 10 meters. Here is the is the first test item:

And the analyzer results:

As you can see, on 20 meters and above not only the SWR is low but also the impedance is constantly around 50 ohm. No need for a second knob! On 30 and 40 meters, instead, conditions deteriorate considerably but I never expected any good performance from such a small loop on those bands anyways. Below 14 MHz I generally use my loop to listen only. It might happen to make a contact once in a while, but it is rare.

The loop responded as I wanted it to… But was it performing? To find out, yesterday I did the first field comparison of four different foldable loops:

1) Toroidal Coupled Loop 6FT (the antenna I’m writing about today)
2) Army Loop 6FT (the antenna I described last time)
3) Inductively Coupled 6FT Loop (classic design, just smaller than usual)
4) Inductively Coupled 11FT Loop (the classic loop! Practically a photocopy of the Alexloop)

Testing was performed on 20 meters WSPR by sending 3 frames on each antenna starting at 7PM local time. By the time I finished with the last antenna it was already 8PM and it’s possible that during such time propagation conditions evolved. The ideal way to perform this test would be transmitting with all the antennas at the same time, but I am not equipped to do that… At least not yet!
Finally, the gathered data:


As expected, the test shows the 11FT loop constantly being on top of the game. Loop dimensions are critical and on 20 meters there is a considerable difference between 11 and 6 feet circumference. On the other hand, the 6 Feet loops (Toroidal, Capacitive and Inductive) seem to be delivering similar performances. There are no clear winners or losers, which is good news because that means I can keep using the latest version (toroidal) which is smaller and simpler to use.

Future plans include building the biggest toroidal loop possible, as well as making my 11FT inductive loop lighter and easier to transport.

Best 73’s

Alain K1FM