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Quick comparison between the PA0RDT Mini-Whip and The RA0SMS Mini-Whip

I was recently lent a RA0SMS mini-whip to try out. Earlier this year I purchased new original Mini-whip from PA0RDT. I have used a homebrew copy of the mini-whip I built a few years ago as well. The new one uses an isolation transformer in the receiver interface as does the RA0SMS antenna. I installed each one in succession near where my CliftonLabs whip antenna is, about 5 feet off the ground strung from a plant hanger 80 feet from the house and more from the road. I have a wideband rf choke designed by CliftonLabs on the feedline about 3 feet from the antenna.
The RA0SMS antenna seems to have a lot more gain than the PA0RDT antenna. It really brings up the noise, anything above 20 MHz is unusable in my installation. As I have mentioned previously the noise floor of my KiwiSDR rises above 20 anyway. They both receive well, especially considering their size and ease of installation. Both receive WWVB with the RA0SMS doing a bit better.
I have attached a spectrum image of each antenna, the capture were about 10 minutes apart in the morning here in upstate New York.

73,
Steve KD2OM


Attachments:
http://forum.kiwisdr.com/uploads/Uploader/33/88cac2c1d652c9bbff7de0d0a8346f.jpg
http://forum.kiwisdr.com/uploads/Uploader/d4/fb2ba67abcd036947383c0c8b22919.jpg
WA2ZKD

Comments

  • Hi Steve,

    I'm sorry to say that neither of those two look that good to me. 

    The signal levels seem incredibly low, have you got an attenuator,  splitter or anything else between the antenna and SDR ?

    I tried your SDR but the switching is disabled at the moment.

    I've got a PA2FWM version of the PA0RDT on my SDR plus a Trask design (probably similar to the Clifton Labs) which outperforms it.



    Please try my SDR and see what you think.


    Regards,

    Martin - G8JNJ












  • edited April 2018
    Martin,
    I agree. I think part of the problem is that the antennas are only a few feet off the ground and I believe both use the feedline as part of the antenna. The feedline to a multi-section choke is only three or four feet long. I believe the comparison is valid between the two antennas but not if you compare them to properly installed versions.
    My antenna switch, the way I had it installed caused some noise at some frequencies, so it is not connected, hence no switching. I am in process of fixing it so that I can switch receive antennas again. If the weather ever gets better here in upstate NY I will install the PA0RDT antenna up about 25 feet in the clear with no choke in line., it should perform fine then.
    Overall I still prefer the LF enhanced Wellbrook loop as a receive antenna below 10 MHz. I have listened to your receiver before, it is very good, I will give it a try again.


    73
    Steve KD2OM

    PS: You have a really RFI quiet site as well.
  • edited April 2018
    Hi Steve,

    All good.

    A few points to consider.

    Height above ground does matter, but only up to a certain point. The key factor is the Signal to Noise ratio produced by the antenna, so once you get above a signal level that overcomes the noise floor of the amplifier circuit (or receiver) you don't really need to increase it much further. Extra height can help you move the antenna outside the local noise field of any nearby property, but it can often be better to have the antenna lower down but further away from sources of unwanted noise.

    My E-probe antennas are all mounted at only about 6ft off the ground. Raising the height can cause problems at some frequencies if the support pole (or coax cable) length is close to 1/2 wave long. This is because (as you previously stated) the coax and any conductive support structure form part of the antenna. If the support is close to a 1/2 wave long the amplifier sits at a high impedance node and so it's ground reference is 'floating' which reduces the amount of signal pickup at those frequencies.

    The other issue is that E-Probe antennas have a very high value of feed-point impedance, so any ferrite (or other) common mode chokes you attach to the coax are not likely to be able to provide a sufficiently high enough value of choking impedance over a wide enough frequency range to be effective.  The trick is to use a combination of ground rods (to provide a low value of shunt impedance) and chokes (to provide a high value of series impedance) so that the combination works like a potential divider network and the overall effectiveness is vastly increased as a result.


    image

    My suggestion would be to add an earth spike or ground rod at the base of the support structure and connect it to the screen of the coax at that point. Then add your common mode chokes as required.

    If the signal level is still very low you could try increasing the size of the plate antenna or replace it with something like a 1m 'whip' antenna.

    Regards,

    Martin - G8JNJ


  • That diagram there - the chokes are "as needed" right? Or do you need 2 for sure? Those brown lines above the "ground" symbol. What are they? Thinking of how I could ground coax without making another cut.
  • I can see a lot of what looks like SMPS noise, I think its in both of the spectrum plots, but shows up more on the RA0SMS. I had a major issue with a local SMPS, looks like you need to prove that one isnt under your control before looking further. I can see the issue from around 10 MHz to 30 MHz

    Ta Dave
    M0TAZ
  • edited August 2018
    @Lonecrow

    "That diagram there - the chokes are "as needed" right? Or do you need 2 for sure? Those brown lines above the "ground" symbol. What are they? Thinking of how I could ground coax without making another cut."

    Yes the 'chokes are as needed' you will reach a point where adding more makes little or no difference.

    The brown lines were meant to indicate the noise level being carried on the outer of the coax screen gradually diminishing as it gets closer to the actual antenna.

    The three stepped horizontal bar symbols are ground connections and meant to indicate the addition of earth rod's, which like the chokes are "as needed".

    You can ground coax by carefully stripping back the outer insulation and tightly wrapping some wire around the screen (if it is copper and you can very quickly solder with a hot iron even better) then tightly wrap the connection with some self amalgamating tape and cover with normal PVC insulation tape for a water and UV resistant joint.

    I hope that now makes a bit more sense.

    Regards,

    Martin - G8JNJ
    Lonecrow
  • Since I use RG6 coax with F connectors as the antenna feed, the easiest way to ground the coax is use a CATV grounding block.
    WA2ZKDLonecrow
  • Problem is finding a spot in the middle of your yard to ground it :)
  • Oh and thanks for the explanation G8JNJ makes sense.
  • Firstly, thank you for the helpful info. Secondly, with regards to the common-mode chokes, do you have any photos of how they should be made? I have toroid cores and would like to try to make my own chokes. So, how many windings / turns would be suitable? Thanks.
  • Until more knowledgeable folk arrive... it depends on how low you want them to work and how much RF is about.
    With ferrite cores the affect is square of the turns through the ferrite so if you can get 6 turns then that is a pretty good level of impedance to common mode signals, reverse the direction half way if you can.
    We are seeking to impede the unwanted mode so it's a trade off between required affect and physical build, there is also an advantage to keeping the input coax away from the clean output so 6 turns where there is separation is probably better than 8 where IN is touching OUT, if it requires more impedance add more cores rather than bunch the turns.
    E.G.
    http://www.df9cy.de/tech-mat/cy40mnoisered/cy40mnr.htm

    Here there is a lot of VHF about so even some air-coiled loops helped.
    Stu
  • This web page provides a chart for winding choking Baluns

    http://karinya.net/g3txq/chokes/

    Regards,

    Martin - G8JNJ
  • This is the choke that I use. I have one on both of the KiwiSDR's I have. http://rx.jimlill.com/cmca.pdf
  • I have found it very worthwhile to use toroidal 'choking' in the form of a Guanella balun simultaneously on all lines going in/out of my KiwiSDRs.

    as shown in the bottom portion of the figure.

    Like the situation so nicely described by G3TXQ's graphic, suitable cores need to be used, possibly several of them to cover the broad frequency range of the Kiwi. By winding equal turns in the same direction with each line on a single core, the effective current that can flow through the receiver is reduced by the "transformer action" of the cores. Current that would otherwise flow into and through the SDR creates common flux within the core that causes an opposite current to come out, thus pushing total current toward zero and effectively creating a much higher impedance than can be the case when each line has only a dedicated core.

    From my experience, CM currents through the entire KiwiSDR structure produce IZ drop that appears across the effective ADC input, which is a complex function of PCB layout and conductivities - the "attenuator problem". For this reason, I believe, treating all lines as common mode lines, in addition of course to also eliminating differential noise across pairs of individual conductors of those lines, e.g. power line or inter-pair CAT5 noise, can be very effective in improving the KiwiSDR noise floor.

    Glenn n6gn
    Powernumptysesykes71
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