Advice on sharing RX antennas with 3 SDRs in the most effective way
Currently, we at OH6BG have two kiwis and one QS1R, and they should share the same antennas. I wonder how other kiwi/SDR operators have shared their antennas so that the impedance matching, performance etc. is as good as possible. We do have a 10-dB preamp before splitting the signal to two RXs, but also have found that a simple T-connector may not be the most recommended way, and now a third SDR will complicate things a bit further. Any advice/pointers on how to handle antenna sharing with three receivers?
Thanks in advance!
73 Jari OH6BG
Thanks in advance!
73 Jari OH6BG
Comments
Manual with data: http://sdr.eladit.com/ACCESSORIES/ASA-15/datasheet asa15.pdf
It's more than you need, but it performs well and covers from 9 kHz - 70 MHz. I think it has now been superseded by the ASA-16.
I don't have personal experience of that device because I figured for me it was cheaper to add more LZ1AQ amp and loops.
For example the Elad unit specifies an output IP3 of >24dBm, which is not particularly good, and would be typical of something using a PGA-103+ or similar as the active device which will provide an IMD performance of around an IOP2 of +50dBm and IOP3 of +30dBm.
My suggestion would be to use something like the DX Engineering RPA-1 followed by a Mini-Circuits splitter.
I've built a copy of the RPA-1 and it works well from about 10KHz to 40MHz and I have measured the IMD performance as an IOP2 of +92dBm and IOP3 of +45dBm.
Details are on my Active antennas web page.
https://www.g8jnj.net/activeantennas.htm
For GPS splitting you can use active L-Band splitters that were originally designed for distributing satellite TV services.
https://cpc.farnell.com/global-communications/split4af/4-way-active-splitter/dp/SE00009
I used this one but modified by cutting some tracks and adding an internal regulator so that it provided +5v out to feed the GPS antenna
Regards,
Martin - G8JNJ
With Zo = 50 Ohms, this simple formulae to calculate the resistors required - > R=Zo x (N-1)/(N+1), where N is the number of ports required...
Sure this type of splitter results in splitting loss, but it maintains a flat frequency response right across the full frequency range and the gain deficit can be made up ( calibrated ) by adjustment of the Kiwi S-meter calibration (dB) & Waterfall calibration (dB) on the ADMIN Config Page.
The WYE Splitter can be used on the SDR inputs, because it is completely unnecessary to have receiver to receiver isolation ( as there is no local oscillator to cause inter-receiver hetrodynes / beats & birdies )
73'
Paul
VK3KHZ
www.amradioantennas.com
If your location is rather noisy to begin with, then there is no need to do through this trouble: This can be determined by tuning in a "blank" spot on 10 meters and observing the S-meter reading with and without (preferably substituting the antenna with a 50 ohm load) and seeing if there is an obvious difference - at least a couple dB, consistently higher with the antenna connected. If there is no obvious difference, your receive system is "gain starved".
I have found that with the Kiwis at the Northern Utah WebSDR - with an antenna that is essentially "flat" from 3-30 MHz (around 6dBi peak across this range), split four ways, around 20-ish dB cumulative signal gain is required to assure that the KiwiSDR can reliably "see" the thermal noise floor on 12-10 meters at this fairly quiet site - but there's a caveat to this: If you put this much gain in front of the KiwiSDR "naked", you will overload it on lower-frequency signals - that is, those below 12 MHz or so - when the bands open at night - and the receiver will overload badly, much of the "A/D" capacity being spent on a couple dozen strong signals at the expense of everything else.
For this reason, several in this group have devised "limited attenuation high-pass filters" where those lower-frequency signals are attenuated by a certain amount - something in the 10-20dB range: Enough to knock those very strong signals down significantly, but not enough to put the receiver below the (increasing inversely with frequency) natural noise floor.
You may peruse that thread here: http://forum.kiwisdr.com/discussion/comment/4490#Comment_4490
This allows amplification of the signals across the HF spectrum of a sufficient degree while considering the dynamics of those signals - and the dynamics of the KiwiSDR's A/D as well.
In the current configuration at the Northern Utah WebSDR, the signal path is along the lines of:
HF Antenna -> Lightning Protection -> 2-way splitter (the KiwiSDRs are on one of these branches) -> Adjustable AM-reject filter network -> High-dynamic range 14dB RF amplifier -> 2-way splitter -> Limited attenuation high-pass filter for Kiwis -> High-dyanamic range 14dB RF amplifier -> HF+LF Combiner -> 4-way splitter -> KiwiSDRs
A (slightly outdated) block diagram and more information about that signal path may be found here: http://utahsdr.org/info/rx_rf_distribution.html
The amplifier is described on this page: http://utahsdr.org/info/rx_softrock.html (See Figure 3 on this page and the other, similar amplifiers embedded in the diagrams of the other receivers.)
Note that the system at this WebSDR has been designed to use both narrowband "softrock" and wideband direct-sampling receivers, hence the two complete signal paths. The rationale for this was to "future-proof" the system: The narrow-band "softrock" receivers are still the best way to inexpensively get high performance, but the "wideband" receivers (KiwiSDRs) are still not suitable for many dozens of users - and the combination of affordable, very high performance hardware and the software for sharing many users on wideband receiver hardware is still a little ways off - but we expect it to be available in the next couple of years.
73,
Clint
KA7OEI
An example of a simple bandstop & notch filter is on the splash screen at http://n6gn.no-ip.org:8073/ which is at my home QTH. Measured results closely match the simulation which is shown. Parts cost including a small PC board for which I can provide a layout is less than US$10.
73 Jari
The only caveat is that both Kiwi's have to be powered on, turn one off and it kills the GPS reception for the other.
The other use for those types is as high pass filters for the HF side, I was feeding a mast mounted vertical into the HF side with a ground mounted mag loop for under 10MHz.
I have 6 devices running on one GPS antenna, a 40 dB gain unit
Downside - sticky-out dc feed but that's about it.
You are right of course, I just like buying things that can be reused.
The DXE preamp is unfortunately not available anymore and I couldn't find a commercially available alternative.
I've found several active antenna splitters, but none of them has particularly good RF characteristics (IP3, noise figure, 1dB compression point). There's just one from which I haven't found any data and/or the producer hasn't replied yet; the Elad ASA-16. Anybody got data on this? Or an alternative?
The one's I've already checked are:
- LNA4HF (LNA only)
- Cross Country Wireless HF Multicoupler
- AAS-300
- Elad ASA-15
- ARR P0.1-30/20VD
EDIT: I just found that the DXE preamp has a successor, the DXE-RPA-2. At least the specifications are better than those of the other preamps.Thanks, 73
Stefan
Woofferton is producing strong signals here, often -20 to -10 dBm on multiple bands.
I have asked 9 shops and none of them could sell me one, including mouser.
I pay in advance.
Personally I'd keep an eye on Ebay, as there are always lots of splitters up for sale.
Note that the 75 ohm ones work fine too. For example I use ZSC-2-1-75 (2 way) and ZSC-4-3-75 (4 way)
Here are some currently on offer - make sure you check the frequency range, especially the lower limit, on the Mini-Circuits website before you buy.
https://www.ebay.co.uk/itm/Mini-Circuits-ZSC-2-1-75-Splitter-BNC-Connectors-75-Ohm-Used/362439249988?hash=item5463101044:g:tQoAAOSw64tbolK2
https://www.ebay.co.uk/itm/Mini-Circuits-Splitter-ZSC-4-1-75B-75-Ohms-BNC-Connectors-RF-Microwave/264303124710?hash=item3d89b1a0e6:g:6YEAAOSwH-daHSUe
https://www.ebay.co.uk/itm/Mini-Circuits-Part-Number-15542-Splitter-Model-ZSC-2-1B-Unused-Old-Stock/312310591710?epid=1038679278&hash=item48b72970de:g:Q1cAAOSwLN5WlTVX
Regards,
Martin - G8JNJ
You could try building these designs
http://f6aoj.ao-journal.com/crbst_136.html
BN73-202 with four turns on the second transformer rather than two (to improve the LF performance), the -1dB points are at 20KHz and 70MHz.
or
http://www.dxing.info/equipment/rolling_your_own_bryant.dx
or
BN73-202 7 + 2 x 5 turns = 3.1dB loss plus -1dB down @ 20KHz & -1.5dB @ 30MHz
I twisted the wires (I use PTFE or Kynar covered wire wrap, insulation not particularly critical) with an electric drill, about 4 or 5 turns per inch. This improves the coupling between wire pairs and halves the time taken to thread the wires through the holes in the binocular cores.
I've also got a design for a simple 4 way split that I built somewhere. I must dig out my notes and try to write this up properly.
Regards,
Martin - G8JNJ
Note that all 4 way splitters have a minimum of 6dB through loss, regardless of whatever is connected, so you should only use them after a suitable amplifier with about 10-12dB gain.
All the designs I tried tended to be quite difficult to setup correctly, and this would be especially problematic if you didn't have access to suitable test equipment.
Problems include poor match and lumps and bumps in the frequency response.
However if you want to build something simple that works, this is about as good as it gets and should only cost a few dollars to construct.
Input and output match is typically better than 1.5:1, the only downside is that the port to port isolation is worst case 12dB.
The more complicated designs can achieve better than 20dB, but use two more transformers with different winding ratios, have greater losses and are more difficult to get a good frequency / amplitude response.
The choice is yours :-)
Regards,
Martin - G8JNJ