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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


  • edited March 2019
    I use this:

    Manual with data: 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 I think it has now been superseded by the ASA-16.
  • Thank you for the suggestions! Really need to dig deeper into this :smile:
  • edited March 2019
    Another option maybe worth considering?
    I don't have personal experience of that device because I figured for me it was cheaper to add more LZ1AQ amp and loops.
  • Jari, I don't think you need to dig deep. Mini-Circuits have 1:2, 1:4 and 1:8 (and maybe more) splitters of excellent quality. Combine with preamps if needed. Picture shows four 1:8 splitters + RF Engineering preamps at my KONG location. A total of 15 SDRs of various guises and four antennas are supported, though all may not run at the same time.
  • If you want LW and MW, be conscious of the low end cutoff on any scheme you use
  • Be careful regarding some of the commercial active splitters. Their IMD performance is not that good as they often use MMIC chips that can only achieve modest performance.

    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.

    For GPS splitting you can use active L-Band splitters that were originally designed for distributing satellite TV services.

    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


    Martin - G8JNJ
  • edited April 2019
    Use a WYE splitter, do a google on

    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 )

  • be careful with filters between the splitter and RX. Depending on type, they may not be isolated by a resistive splitter.
  • Yep, I put filters ahead of the splitter... Q.E.D.
  • edited April 2019
    It's worth mentioning that at a quiet location, even a KiwiSDR connected directly to a (nominally) unity-gain antenna is not quite sensitive enough to hear the IRU-defined noise floor - much less if it is split several ways. What this means is that unless some rather rugged amplification is included in the signal path, the KiwiSDR will not be able to hear signals near the thermal noise floor on the higher bands - say, >=20 MHz or so.

    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:

    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:

    The amplifier is described on this page: (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.


  • I think Clint brings up an important point. For a good installation we are trying to optimize dynamic range with the low end limited by propagated noise. This requires protecting the top of ADC level from local strong signals and is location/time/antenna/preamp dependent. Generally, using notches and bandstop filtering helps this without penalizing sensitivity.
    An example of a simple bandstop & notch filter is on the splash screen at 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.
  • Just a quick follow-up here: We now have a comfortable setup for feeding two Kiwis, one Red Pitaya, and still have a place for another one :) Speaking of GPS, we managed to get a good place for the GPS antenna outside the shack so there are good 10-12 satellites visible at any time. This means that we could be able to share that GPS signal with both the Kiwis. However, do you see any problems if we just share the GPS signal by means of a simple T splitter? Are there problems with voltages? Both Kiwis are powered by the same Power Supply Unit.

    73 Jari
  • I asked the same question and the reply was along the lines of "no problem sharing GPS by T splitter".
    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.
  • edited May 2019
  • Looks like a good device but also maybe consider the cased Bias Tee devices on ebay, in the UK a metal cased 10MHz-6GHz bias tee is cheaper.
    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.
  • The GPS antenna needs bias but only from one device, kiwi or whatever. Regardless of what splitter you chose, one "leg" needs to provide DC to the antenna and the other legs should be blocked. If you antenna other non-kiwi devices to a splitter with blocks, some of them detect an antenn antenna and those need 200 ohm resistors on the blocked port to assure they know they are on an active antenna.

    I have 6 devices running on one GPS antenna, a 40 dB gain unit
  • A bias tee without feed is just a DC block no? Has more than one use, costs less (here), metal case.
    Downside - sticky-out dc feed but that's about it.

    You are right of course, I just like buying things that can be reused.
  • These would work too and the 200 ohm reistor could be added on the DC feed point if needed.
  • Thanks for all the comments, appreciated! In our case, we decided to use a simple T splitter to share the GPS signal with two Kiwis (the two Kiwis are powered by the same PSU). This setup worked for us, but your mileage may vary. Please read the comments from WA2ZKD before anything.
  • edited July 2019
    I'm also trying to find an efficient way to split my antenna across 3 receivers.
    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
  • What kind of antenna are you trying to split?
  • edited July 2019
    Inverted-L Antenna 8x25m (hopefully soon to be 12x40m).
    Woofferton is producing strong signals here, often -20 to -10 dBm on multiple bands.
  • Mini-Circuit makes some stuff that would work and you can often buy them surplus on eBay quite cheaply. For an amp, I use a ZFL-500 and for splitter ZSC-3-2+
  • Does anyone have a possibility to order a Mini-Circuit ZSC-3-2+ for me? Or does anybody know a shop that sells to private customers?
    I have asked 9 shops and none of them could sell me one, including mouser.
    I pay in advance.
  • edited July 2019
    You can usually buy directly from Mini-Circuits - but they often have a minimum order quantity and you have to wait several weeks for delivery.

    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.


    Martin - G8JNJ
  • I haven't tried it but I suspect that you could also build one for yourself very cheaply. If one examines one splitter schematic, it would seem that 4 cores,connectors and some wire might do the trick over some or even most of a kiwi's frequency range. I'd think it should be able to make one from individual transformers as well.
  • Has been taken care of. Thanks @ Jim
  • Hi,

    You could try building these designs

    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.



    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.


    Martin - G8JNJ
  • edited July 2019
    This afternoon I played with various 4 way splitter designs.

    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 :-)


    Martin - G8JNJ
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