Further to Wellbrook ALA1530LN connection to HI-Z port

Useful information regarding antennas for SDR products.
Paul
Posts: 242
Joined: Fri Feb 17, 2017 9:52 am

Further to Wellbrook ALA1530LN connection to HI-Z port

Postby Paul » Mon Apr 23, 2018 9:29 pm

If I understand the following correctly, it should be possible to connect the ALA1530 interface output directly to the Hi-Z port without modification, or can someone tell me whether I am mistaken?
Ports A and B are intended as general purpose RF ports, but the Hi-Z port is really intended as the
prime port for HF and below and has been optimised as such. This is one of the major changes in
thinking from the RSP1. The reasoning here is that you seldom get a single antenna that gives
optimised performance from VLF to UHF, and so the majority of people will tend to use separate
antennas for HF and VHF/UHF. We chose a 1 K ohm impedance for this port, to simplify the
connection to a random wire antenna which is the antenna of choice for HF for many of our
customers and is also a good choice for frequencies below 1 MHz. Because of the long
wavelengths involved at HF, this port also works well for 50 ohm antennas as long as the feed
cable is not excessively long, but for long feed cables, you can simply use a reverse 9:1 balun
between the Hi-Z input and the end of the feed cable to achieve an adequate match to 50 ohms
for this purpose. When operating below 1 MHz (down to VLF), though, the direct connection will
work best, even with a 50 ohm feed as the high impedance termination will deliver the highest
terminal voltage at the input to the RSP2 and reflections will really not be a problem because the
wavelength is so long

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g1hbe
Posts: 354
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Further to Wellbrook ALA1530LN connection to HI-Z port

Postby g1hbe » Mon Apr 23, 2018 10:29 pm

Yes, you can connect the coax output from the Wellbrook interface box directly to the Hi-z input. Fit a link between N & G and connect the coax with the inner to P and the screen to G. I used mine like this for some time and it worked well.
However, in my quest for lower noise levels I later made a 1:1 ferrite transformer to fit between the coax and the Hi-z port. The reason was to break any ground loops that might be injecting noise into the system. It worked, with hash levels dropping by 10 to 15 dB depending on frequency. Greatest effect was at LF.
I wired this with the coax going across one winding (the primary) and the other winding (the secondary) feeding across the P & N terminals with the G not connected. Note there is no connection between the pri & sec.
In theory there should be more turns on the secondary than the primary to match the 50 ohm Wellbrook to the 1k of the RSP2, but I found it made little difference.

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Andy

Paul
Posts: 242
Joined: Fri Feb 17, 2017 9:52 am

Re: Further to Wellbrook ALA1530LN connection to HI-Z port

Postby Paul » Wed Apr 25, 2018 6:24 am

Thanks Andy
I now have a successful Hi-Z connection to a 50 ohm feeder from a mini-whip, whilst I wait for the loop arrival.
I suppose there must have been a good reason to choose that type of plug...
Mr Iken appears to be inundated with orders (can't be bad for him).
I do appreciate your taking the trouble to bother with what must seem like trivia.

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g1hbe
Posts: 354
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Further to Wellbrook ALA1530LN connection to HI-Z port

Postby g1hbe » Wed Apr 25, 2018 2:43 pm

Not at all, Paul. I hate to see newcomers put off by problems that actually have simple answers if only someone would take a few minutes to explain. Andy Ikin told me he's slowing his business down now he's 'getting on a bit', but it looks like the advent of cheap SDR's has caused a surge in demand!

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Andy


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