Technical Topics > Equipment
Using ATU with SDR or Portable Radios?
RobRich:
Agreed on the inputs, as noted earlier, portables tend to be high impedance. It is cheaper to just dump the attached whip and the external antenna jack into same frontend. Kind of like lots of portables skip on even basic $0.25 ESD protection diodes.
I have used symmetrical feedlines for various antenna projects, though just from a convenience standpoint alone, I suspect there are far more SWL enthusiasts here with dedicated MW/HF antennas running coax instead of twin-lead, ladder-line, or similar. ;)
Either way usually even a basic outdoor antenna has more than enough gain for a portable regardless of whatever feedline impedance characteristics barring a short, and even then that might work depending upon the frequency.
We know he has a seemingly working active antenna available, so that is a potential place to start before delving deeper into antenna design. Assuming it is not the active antenna preamp itself overloading, adding attenuation is the usual next basic step to deal with frontend overload.
Coax is present so I lean towards us starting with readily available and affordable attenuation options; be it a variable attenuator or stacking fixed attenuators if need be. Various F- and SMA-connector models are available for a few dollars at a variety of online vendors and electronics shops. Add a couple of cheap adapters if needed to insert the attenuator(s) between the bias tee and radio. Alternatively, three resistors can do the same if one does not mind a little soldering DIY job.
https://www.everythingrf.com/rf-calculators/attenuator-calculator
That said, this is not exactly ideal....
--- Quote from: alpard ---miniwhip was working without amplification - as a passive antenna
--- End quote ---
....as Kirchhoff's current law applies. ;) So just like any other dipole antenna, even if your setup does not look like a traditional one, you could consider the radio circuitry (plus any electrical grounding, plus possibly the whip antenna, etc.) as one side of the antenna, and the coax running to your un-powered active whip as the other side of the antenna. Lots of YMMV there. It very well can and should receive signals, but I can imagine the resulting SNR is not great.
alpard:
My Active Miniwhip works fine on say 6 - 15 Mhz with the preamp powered on.
But under 5 Mhz, it overloads the radio. I have to switch off the preamp for 4 - 5 Mhz RXing with the Active Miniwhip.
So it depends on which frequency it is, the Active MiniWhip overloads or works great.
I also found that there is no one antenna which works well for the whole band or MW LW SW, unless it is active loop such as Wellbrook or MLA30. And even these active loops, there will be portion of the band, where they are not good or medium performer.
I have 2x YouLoops, 1x Active MiniWhip and Random Wire, and they all work better for one part of the band better than the other bands.
I might go and DIY simple L-match tuner, because I have a few old variable capacitors lying around in the loft. All I need would be a plastic pipe and enamel wire to wind coils to make inductor.
alpard:
Ended up ordering an ATU.
I try not to amass radio gears, but needs must.
Hopefully it will give extra front end and pass band filtering effects which will help curing the nasty intermodulation problems on 4-5 Mhz.
But I am not sure on the result until tried and tested with the newly ordered ATU.
alpard:
Tried the ATU with my portable radios and random wire. It worked great. Yes, ATU works well with the portable radios.
It peaks signals from the noise. It seems helping curing the MW band signals overloads too. If one is using wire antenna and SW DXing, ATU is a must, be it portable or tabletop radios.
Ended up getting a vintage Marconi Attenuator as well, but it is for 600 ohms. It might help work better for longwire into the Hi-Z.
Josh:
Project IC-R71A recently made me deploy a GROVE TUN-3 Minituner for the first time in years. All I wanted to do was some quick ad hoc testing on the R71A, string a few feet of wire over a window frame and see how well the combo did.
The TUN-3 is a simple passive device, you have two controls, one sets the range and the other tunes. Have to say it wrung every drop of rf outta that 8ft piece of wire, amazing. Of course these work best below about say 15MHz but they can still help in keeping undesired rf out of the system, the lower the freq the better the q I suppose. I don't recall the Drake R8 I last used the TUN3 with as having so profound effect on me, but that was with a 70ft sloper from the second story down to the mailbox at the end of the driveway, much more rf to begin with in the R8/70ft sloper/TUN-3 setup than what one gets from 8ft of wire strung over a window sill feeding an R71A. The mail folks thought the sloper wire to the mailbox was for earthquake detection, heh.
Order a Grove TUN-3 MiniTuner today!
https://worldradiohistory.com/Archive-Monitoring-TImes/Grove-Catalog/Grove-1989-02.pdf
Also be sure to check out the new Fabulous Grove SR1000 Professional Surveillance Receiver! ! !
I wholeheartedly recommend any suitable atu device for rx use when one wants to focus on a specific channel. And by suitable I mean one with manual controls. Most any new or used manual HAM transmatch will do, however don't expect the HAM models to tune below 1.8MHz, a lot of them only go down to 3.5MHz, so one made specifically for rx use would be best suited for lower freq excursions.
Or build your own;
https://aa7ee.wordpress.com/2021/03/23/a-passive-tunable-hf-preselector/
https://www.eevblog.com/forum/rf-microwave/lets-start-a-project-to-build-an-hf-preselector/
https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qex/2003/03_04/page45/index.html
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