Thursday, 20 May 2021

SRH 789 (clone) antenna mod for hackRF Portapack

SRH 789 antenna modification

The Portapack

The Portapack (in my case, the H1) ist a great addition to the hackrf one. After I had damaged two hackrfs, my current setup now uses the version with a modified RF-frontend by Clifford Heath. This now works safely alongside my ham radio gear.

My portapack setup
It covers all ham bands from 160m to 6cm (any anything inbetween). In practise, it starts becoming useable at 80m up to 23cm. Beyond that, I don't own suitable antennas and/or preamps / filters.

VHF contest

Although the latest version of the Mayhem firmware now supports SSB-TX, I would not stand much of a chance in a contest with the portapack as main rig. But my trusty old 1980s FT-290 that I still like to use for portable operations, does not offer any of the luxuries that we came to expect from modern transceivers. So I like seeing the section of the band I am working on, on the portapack.
Not so this time. The strongest stations that were 59plus on the FT-290 barely showed up on the spectrum view.
The HackRF is not the greatest receiver there is with it's total lack of band filtering or preselectors and I suspected the nearby FM Radio tower to interfere.
The same disappointment a little later, when I was out, hunting a radiosonde that practically should have been on top of me. Something with my setup was very wrong.

The Antenna

I often used a 2m/70cm dual band antenna on the portapack, but a while ago I got a cheap clone of the Diamond SRH789 antenna, which should have been more than good enough for the job and does not poke through my backpack.
Not the original SRH789

So back home in the shack, I checked the SRH 780 against the dual band antenna and it became quite obvious that the problem did not lie with the HackRF this time.
Having nothing to loose, I took the antenna apart and was surprised to find a totally pointless plastic piece between the center connector of the SMA side and the telescopic whip. The copper wire that had once bridged the plastic piece had torn off when I rotated the antenna.
The antenna is obviously not meant to be treated that way, but with the portapack sitting on the ground, I need the antenna to point up, of course.
There is absolutely no L/C matching RF-magic going on in the antenna base. So there is no point in keeping that plastic bit there.
No RF will pass this
Fortunately it has M4 threads on both ends, so an M4 screw will be a simple, more RF friendly replacement.

M4 fits just fine. The copper wire can be removed
If you want, you can yank out the glued in center conductor, so the aerial can spin more freely. 
With a sharp pull, the center pin of the SMA connector comes loose

When I put it all back together, I took the antenna through it's paces with the NanoVNA. Not surprisingly, I got mixed results. This is to be expected from an antenna that is nothing but a piece of iron rod sticking out the back of the transceiver.
Radials? Counterpoise? GroundPlane? Nope.
The resonant frequency varied wildly with the surrounding conditions. i.e. me touching the VNA's case or leaning over my workbench. With some tuning you'll get an acceptable SWR all through the 100MHZ zo 1.1 GHz range. The impedance is a different story: While somewhere around 50 Ohms for 2m and 70cm, where the antenna works as a quarter wave, the impedance on the higher bands is all over the place.

Final word

This is not a great antenna. You can't expect miracles. Possibly because it is a fake/counterfeit product.The original Diamond antenna may be a better choice. Please leave a comment if you have one.
As a mainly RX-antenna with superb portability it is acceptable, but no match for TX-capable wideband discone (that will be significantly bigger and more expensive).



Thursday, 25 February 2021

Iphone 12 hearing aid compatibility problem

iPhone 12 bluetooth hearing aid compatibility issue (and a "works-for-me" solution)

More on that issue, including audio samples shortly on my YouTube channel.

No problems with iPhone 8

I had an iPhone 8 for the last 3 or so years, and just by the end of last year, I upgraded my hearing aid to the latest&greatest Li-Ion powered bluetooth connected version I could get my hands on.

Specs & hearing aid - no problem.
Things become tricky with a mask.

Apart from better hearing, the additional bonus of having a top quality, hands-free, near invisible headset for the numerous audio and video conferences that came with the pandemic was just a blast.

Until I switched to an iPhone 12.

The hearing aid

It seems that there is only a small number of actual manufacturers and a lot of brand names hearing aids are sold under. Mine are sold as SoniTon UP5 R Li with a CE 0543 mark that indicates it has been certified as a medical device in Denmark.

The actual manufacturer seems so be Unitron Hearing Ltd. in Canada. Other names/brands the device is sold under are: Vista DX 350 R Li, AudioNova DX 30 R Li, DX Moxi Move R 3, scala DX 350 R Li, excellence 2 V2 R Li

It is not specifically "made for iPhone", but can be used universally, so while the use as headset/headphones is generic, control is via Bluetooth LE and a "hearing remote" app.

The "hearing remote" app allows control via the iPhone

iPhone 12 problems

At first I didn't notice anything wrong. Podcasts and music were fine to listen to. But callers and team-mates complained about choppy, scratchy audio. Same thing on my side: distorted audio.

I went investigating and found that to be a common problem for people using the iPhone 12 series (mini, pro and "regular" alike). Even for hearing aids specifically "Made for iPhone. The sterotypical recommendation to use the latest iOS version as found on Apple's support and discussion pages were useless for me. Apple claims to have it solved for "Made for iPhone" with iOS 14.2.1, which left me on my own.

The solution

It appears, that the problem lies with "Adaptive Bluetooth". I am not sure which flavour of "Adaptive Bluetooth" that is, since both "adaptive frequency hopping" and "adaptive codec selection" (Qualcomm aptX Adaptive). If you know, please leave a note in the comments.

Disable Adaptive Bluetooth
Adaptive bluetooth can be disabled in the "remote hearing" app. The hearing aids have to be restarted after changing that setting.

With this setting changed, calls, voice recordings and video conferences are ok again. I am not quite sure whether or not I sense a very slightly reduced audio quality for music since then. It so, the codec chosen for compatibility might not be the best for music.