SRH 789 antenna modification
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.
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.
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.
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).