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Antenna Recommendation

Hello,

Any antenna recommendations for use with KiwiSDR? I just want an antenna that works well through LF, MF and HF and inexpensive (less than $250), and space friendly as well. TIA!

Comments

  • Can you tell a bit about your environment?

    Can you place an antenna far away from buildings, or are in an apartment of a 20 story building, etc.?

  • edited February 20

    @HB9TMC I'm in an urban environment, with electrical wirings on the poles. But the transmitters are 20-25 miles away from my location, so I think that wouldn't cause any issues. I am living on a house (inside subdivision) with no front yard or lawn and HOA is a bit restrictive as well, but I can mount a small pole for antenna though.

  • Whatever you use, it is likely to have to be a bit of a compromise.

    Active monopole / E-Probe - Very small and easy to hide, good performance from VLF to 30MHz + Omni-directional. Being an unbalanced high impedance antenna, it is very prone to noise pickup on the coaxial cable, which actually forms part of the antenna. With care this can be minimised, but it is very difficult to completely eliminate it. PA0RDT, PA3FWM, and clones.

    Broadband loop - can be ground mounted and hidden or disguised. It is directional, with a sharp bi-directional null, which can be useful to minimise specific interference sources. LZ1AQ, Wellbrook clones and Cross Country wireless are ones I've personally found to be good performers.

    Broadband Active dipole - similar to the broadband loop, but needs to be mounted above ground and ideally kept away from other conductive objects. I can be mounted vertical for omni-directional reception, or horizontal for bi-directional coverage. It is another antenna that is easy to disguise as something else, but care needs to be taken with its placement. Old Datong AD270 / AD370 are very good, if you can obtain one second hand.

    I also know that some folks on this forum make various active antenna PCB's, so if you are interested in building something yourself, let it be known, and I'm sure they will reach out to you.

    If you wish to try a few different antennas in a typical urban environment, then visit my home KiWi which I tend to use as a test bed for different ideas.

    http://g8jnj.zapto.org:8073/?ext=ant

    Antenna 8 is mounted inside my loft and is intended to show how much worse noise pickup is when the antenna is close to other electrical devices. Antenna 9 is just the coax running to the remote antennas, with nothing connected, and is intended to show the effect of ground loops.

    This is a fairly recent photo of some of the antennas clustered together, and partially hidden by some bushes. I have had to adjust the contrast to try and make it easier to see them.

    They are probably not performing quite as well as they could be, as the weather has been very wet and windy, and I'm waiting for a good day before I can check that nothing has been disturbed.



    Send me a PM if you need more information.

    Regards,

    Martin

  • Although less-than-optimal, about 9 metres of 2.5 mm^2 mains wire is all I use. Or two 9 metre pieces - makes a fantastic DFing tool as shown in the pic of a hotel in Taiwan in the third link, or stacked vertically to get a bit better S/N when I'm in Hawai`i.

    73, VR2BG.

  • Hello Jolo,

    here I used Apexradio 303WA-2, which is a easy-to-install passive antenna , so it doesn't require electricity and easy to maintain. The price is within your acceptable range, even import tax included.

    the antenna can be very concealed if installed at high position....

    However, the disadvantage is that online shopping in Japan requires a little effort on your part and waiting. The SNR is also a bit lower than the active antennas, and it is difficult to rank in the top 20 even at night. 😅

    For instance,

    check my reception below, it is much better at 5~11 PM (UTC+8)

    21047.proxy.kiwisdr.com:8073/

    what it looks like when installed ...


  • Hi Jimmy,

    I was intrigued by your passive whip antenna, the ApexRadio 303WA-2.

    I did a bit of research online and managed to find some notes regarding its construction.

    It is basically a 1.7m long whip, connected to a 64:1 ratio broadband Unun transformer.

    The Apex transformer printed circuit board looks like this image I found online.

    It is 4 x 8 turns = 32 turns secondary (antenna side) and 4 turns primary (50 ohm side). The transformer core is a dark green colour high permeability material that is commonly used as part of the mains filter network in compact Switched Mode Power Supplies.

    This is my very quick copy. The capacitor is simply a DC block, fitted as a precaution, as the rest of my antennas are powered. I didn't bother with separate windings, but simply tapped the main winding part way along, so it is configured more like an auto-transformer. With just 4 turns of wire on the lossy core material (as is the case with the primary), it presents approximately 50 ohm impedance from 1Mhz to 30Mhz.



    This clone is temporarily connected to my home KiWi and antenna switch.

    http://g8jnj.zapto.org:8073/?ext=ant

    In my installation, the signal levels are approximately 20dB lower than from the other nearby active antennas, but this is as I would have expected.

    This is partially because it is ground mounted, and the performance would be considerably better if mounted high up on the side of an apartment block. This is because the differential electric field between the antenna and ground is likely to be much greater.

    Regards,

    Martin

  • A few years ago, I built a couple of these "WellGood" loops. Based off the legendary Wellbrook ALA1530. It's been a pretty decent performing loop. It handles being in close proximity (~4 miles) to a 50kW blowtorch on AM, in addition to other strong local signals. I built mine by hand on perfboard, wound my own transformers (looks challenging, but not really), however George is now offering prefabricated amp boards with bias tee for a very affordable price. You'd need to install the amp in your own weatherproof box, source you own loop material (most people use coax or copper tubing), power supply, and feedline...

    https://www.george-smart.co.uk/projects/wellgood-loop/wellgood-loop-history/

  • Thank you all for the helpful antenna recommendations!! I really appreciate it. I'll take a look on each one of them. Thank you again!!

  • Hi Martin,

    Due to the tight and solid structure of 303wa-2, it is almost impossible for me to get any answers without using a machine tool to disassemble the working part.

    I once referred to the exploration by several Japanese radio enthusiasts, hoping to understand its design principle, but the results are still confusing. The initial consideration for the design of this antenna seems to be that it must connect a coaxial of at least 10m long, but when measuring such a whole, the relationship between its impedance and frequency is oscillating. At this point, I felt that has exceeded the boundary of my knowledge. It is best for me to read relevant books first.


    The analysis circuit you provided here is truly the closest answer I have ever seen to this antenna.


    Best regards,

    Jimmy

  • Hi Jimmy,

    Thanks for those new links, most useful.

    In one of the Japanese links, someone has tried to measure the impedance match, but they have just connected the antenna directly to an antenna analyser, without the coax cable bing present to provide a counterpoise or ground plane, so the results are pretty meaningless. However, it has allowed me to check that my version produces similar result under such conditions. Which gives me greater confidence in my copy.

    Asa result, I have been further experimenting with different core materials and turns ratios.

    The Apex 303WA is very similar to the shortwave section of the AOR SA7000, although that has a 30MHz diplexer to add in a second antenna for coverage on the VHF and UHF bands. But The transformers used in both models are pretty much identical.

    Based on my initial design estimates, I have now figured out how to improve the overall performance of my version. This has required a different transformer ratio, an equalisation capacitor and using a thicker radiating element, all of which seem to have produced about 10dB improvement over my first attempt.

    However, the LF end of the frequency coverage tends to resist any attempts to substantially improve it, or at least not without really degrading the upper frequency coverage above about 10MHz.

    I think the AOR version with a diplexing filter is an interesting concept, and it may be that by splitting the frequency coverage of the antenna into separate frequency bands, it may be possible to improve the LF performance too.

    The weather has been pretty awful again today, but as soon as it improves I'll swap over the existing prototype to the newer version to see if it is any better.

    Regards,

    Martin

  • a longwire + 9:1 balun would work too... could be all DIY or:


  • Settled with the ApexRadio one, thanks again for the recommendations!

  • Hi Jimmy,

    I have now placed the improved Apex clone outside and connected it up to my home KiWi, so try it again and see what you think.

    Generally speaking, the performance in terms of absolute signal levels is about 10dB lower than I'm obtaining from an E-Probe (similar to PA0RDT) mounted at a similar height about 2m AGL.

    With my local noise levels, reception is adequate from 30MHz down to about 8MHz, as the signal level to noise floor levels are about the same. The ultimate sensitivity being limited by the noise.

    0-30MHz spectrum

    Below 8MHz it is not as good.

    0 to 8MHz spectrum


    It is usable down to about 4MHz, where the noise floor and interference is still the limiting factor, but below that the difference in signal levels is approximately 30dB, and only the strongest medium wave broadcast stations can be heard. This is a limitation of this type of antenna, and there is not really much more that can be done with a passive design, other than increase the size of the radiator.

    In your case, I think the antenna height above ground, and the horizontal polarisation may be helping improve performance over the frequency range from approx 1 to 10MHz, and your noise floor is still the limiting factor, especially with the switched mode power supply and data over mains interference you have present.

    If you have a metal balcony and have connected to this to use it as the groundplane / counterpoise for your antenna. You may be able to obtain a slight improvement by fitting a decent common mode choke or galvanic isolator on the coax feeding the KiWi, but I think there is likely to be too much radiated interference from items like the Air Conditioning Units, to be able to make a big difference.

    In summary, I think for your circumstances the Apex antenna is a good choice, and is certainly performing as well as many other KiWi's in better locations.

    Thanks again for leading me along this interesting journey.

    Regards,

    Martin

  • As the weather was a bit better today, I decided to try adding a PGA103 preamp to the Apex clone.

    The signal (and noise) levels are now pretty much the same as all the other active antennas I have connected to my home KiWi.



    I suspect that a lot of the low noise performance, attributed to the Apex antenna, could be simply because the low gain masks a lot of the noise that would otherwise be present.

    It would be interesting to compare the Apex against a more efficient antenna, fitted with an attenuator to make the signal levels appear to be the same.

    Once again this demonstrates that for receive purposes, it is the signal-to-noise ratio that matters, and as soon as your reception is limited by the antenna noise floor, then there isn't much you can do to improve things.

    If your receiver noise floor rises by 6 to 10dB when you connect your antenna, that is pretty much optimum.

    Antennas with directional properties can help, but a lot depends on how much interference there is, and from how many different directions it is arriving.

    Regards,

    Martin

  • Dear Martin,

    Out of curiosity, I just bought a PA0RDT mini-whip clone.

    Then I connected PA0RDT amplifier in series to 303WA-2 to see the what might happen to the variation of Kiwi's SNR.

    It may be because Kiwi has an AGC or automatic attenuation, so there was no surprising finding in the received results. The before and after waterfall will change while the amplifier is switched on/off, but the reported SNR didn't change significantly. Although I thought there was a high probability that this will eventually happen, but I were still curious 😜.


    based on his conjecture:

    https://radio-dxing.blog.ss-blog.jp/2018-06-03

    regards,

    Jimmy

  • Hi Jimmy,

    The PA0RDT amplifier has a very high input impedance, because it is designed to interface with the plate / whip element directly. It acts more like an electronic "impedance convertor" to 50 ohms, than an actual amplifier with gain.

    The matching transformer in the Apex antenna, is performing the same function, but in a passive manner. But there is a bandwidth / impedance ratio implication with the design of such devices, which is why the LF performance tails off.

    With the Apex you need to use a 50 ohm broad band amplifier, that ideally has about 10 to 20dB gain, a low noise figure and good IMD performance. This is likely to require about 100mA current at 5 to 12v.

    Such devices are available fairly cheaply on Aliexpress. But beware of inflated specifications, especially the lower frequency limit, as most of these seem to exceed the device manufacturers specification.

    Something like a Gali-74 device is pretty good down to DC, but there are other similar types. Find out what the amplifiers are using, and then check the device specifications.

    Regards,

    Martin

  • The limitation with many PA0RDT antennas is the size of the plate antenna, which I consider to be too small.

    I've successfully used a 1m long whip in place of the capacitive plate, but I also include a series connected resistor and inductor, to act a a low pass filter and prevent overload from FM broadcast stations.

    If you choose the values correctly, to suit the self capacitance of the whip, it also helps to improve the performance on the upper HF bands.

    PA3FWM has various notes on these topics.

    https://www.pa3fwm.nl/technotes/tn07.html

    https://www.pa3fwm.nl/technotes/tn09d.html

    https://www.pa3fwm.nl/technotes/tn05.html#lpf

    https://www.pa3fwm.nl/technotes/tn08b.html

    If you need help with this send me a PM.

    Regards,

    Martin

  • Hello,

    I agree with Martin. The plate of a Mini-Whip antenna is too small. In my creation, I did not use a whisk, but a large thick aluminum tube, with a diameter of 40 mm and a length of 50 cm, at the end of a stainless steel pole of 3.50 meters high and 50 cm in diameter. The top of the mast is waterproof.

    There is also a filter to avoid the flow of FM stations, in particular the French relay of Audrix (Bergerac in Périgord) is 8 km away and emits a power of more than 1 mega Watts in FM, DAB and TNT. The electronic circuit is equipped with an output transformer.

    On the other hand, I moved this antenna away from buildings with an RG58 cable 50 meters long, and buried it. I have to change this cable, because it has a lot of loss and poor shielding. I must lose 15 dB at 30 MHz.

    The photo shows the antenna.

    The box which is at the base of the mast is protection against parasites that can circulate in the coaxial cable. As well as a reminder of the mass at the base of the mast.

    The result is this : http://sdr.autreradioautreculture.com:8073

    ---

    A second installation (KiwiSDR 2):

    I want to further reduce noise from electrical installations and buildings. The next antenna that I study will be at a distance of 250 meters from the receiver, on a moderately high and clear geographical point. I will use a 75 Ohm coaxial cable, 250 meters completely buried. There's going to be a lot of loss... between 20 and 25 dB at 30 MHz. This antenna will be connected to the second KiwiSDR that I received yesterday.

    Best regards, Philippe

  • As I previously stated, the size of antenna element usually specified for such antennas have only a very low value of self capacitance, maybe less than 1pF. Any shunt capacitance that may also be present, forms a potential divider, and the already very weak signals become even weaker.

    If we assume that the amplifier and antenna mount have a stay capacitance of typically 5pF, then using an antenna element with perhaps 50pF self capacitance (it will also add some stray capacitance to ground which is not useful) should be much better.

    One problem that I failed to mention, is to ensure that the whip is not too close to 1/4 wave resonance on the highest frequency of interest.

    A 'fat' conductor is better than a long, thin one, and something like a 39" / 1m long length of 1" / 25mm diameter metal pipe is probably OK up to 30MHz, and will have a self capacitance of around 50pF.

    Adding a small capacitive 'top hat', or using a tapered conductor (thin at the bottom, fat at the top) can also help increase the useful amount of self capacitance, without substantially increasing the value of unwanted shunt capacitance.

    Care also has to be taken regarding the maximum height the antenna is mounted above the ground reference, as this can also introduce problems with self resonance of the support, which in turn can produce an uneven amplitude / frequency response and radiation angle.

    All E-Probe antennas of this type are very susceptible to noise pickup along the coax screen, as the high impedance makes it difficult to choke off unwanted signals. Burying the coax cable at a depth of several inches or cm, and also adding broadband ferrite cored chokes, is one of the more effective methods of noise reduction.

    Regards,

    Martin

  • Dears Martin, Jimmy

    Martin, I admire the reception quality of your system (http://wessex.hopto.org:8075/); it’s a reference. All is well. Well done. Bravo.

    Subject of the mini-whip :

    actually on my mini-whip antenna, element of 50cm long by 4cm in diameter, we must be around 40-50 pF. Now, I chose the height of 3.5 meters. This is not a coincidence, but a compromise. Below 3 meters, the reception was weaker and I had too vertical a directionality, not good for DX. I did the test on 1.8 meters. Too high, if more than 4 meters, a lot of resonance especially on 11 and 8 MHz and strong reception on 1 MHz. Here is a photo of the interior at the top of the antenna. The electronics are in the waterproof plastic case :

    To cut surface cable currents, I use this device (Mantelwellensperre in German) :

    Then, the cable is buried 20-30 cm underground for about 50 meters.

    Best regards, Philippe

  • edited March 2

    that MMWS is a simple choke.... they can be done with clamp on ferrites thus no interruption in coax



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