Use CAT6 shielded communications cable for antenna feedline, rotor control, and LNA power

KA7UKA7U
in Installation, Antennas and Interference
I have a spool of shielded CAT6 cable and it would seem that it should make a reasonable antenna feed line to the KiwiSDR. Do the wire antenna input lugs work with a balanced feed line? If this type of cable is used and the power to the antenna LNA is on one pair, the RF from the antenna is on another pair, and a rotor control used the other two pairs, would there be a foreseeable problem?
Ron
KA7U

Comments

  • WA2ZKDWA2ZKD
    a look at page 2 here shows the screw terminals are not balanced

    KA7U
  • G8JNJG8JNJ
    Hi Ron,

    The LZ1AQ loop amplifier kit uses CAT6 for power, control and signal.



    CAT 5 & 6 has a characteristic impedance of 100 Ohms per twisted pair. You can build an impedance matching balun or use two pairs connected in parallel and a 1:1 balun. However if you use this method beware, as some CAT cables use different numbers of 'twists per inch' on the different cable pairs in order to reduce cross talk between pairs. This results in some pairs being electrically 'longer' than others., which may cause problems if you connect them in parallel to halve the impedance.

    Regards,

    Martin - G8JNJ
    KA7U
  • KA7UKA7U
    The problems of balance and impedance mismatch might be worth a separate post in this category. I tested the impedance of the shielded loop I've been working with and noticed an interesting set of attributes. The impedance of the loop is higher than 400 ohms over most frequencies until ~22MHz and then it drops and rises back up at ~25MHz. It drops as low as 12ohms. This frequency range is where the amplitude of the 60KHz spurs are most noticeable above the noise floor. The amplitude of the spurs seems to increase as the impedance decreases. Any ideas as to this phenomenon?
    Ron
    KA7U


    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/a2/ef23af060501a4e7369077d0f2a3ae.png
  • G8JNJG8JNJ
    Hi Ron,

    You have got a few parameters interacting.

    With most unscreened broadband loops of about 1m diameter, the predominant loop impedance is mainly a low resistive value. So amplifiers with a low value of resitive input impedance such as the Wellbrook and LZ1AQ (approx <5 Ohms) work well on the LF and MF bands. 

    However as we get closer to the loop self resonant frequency (typically just above 30MHz for a 1m diameter loop) the loop impedance rises and gradually becomes dominated by the inductive reactance, which raises the overall loop impedance. So to get the best results on these frequencies (typically >10MHz for a 1m diameter loop) it's better to use an amplifier with a much higher value of input impdedance (>50 Ohms).

    If you screen the loop you lower the self resonant frequency, so with a typical 1m diameter loop you end up with an impedance dip somewhere in the middle of the frequency range, with the loop impedance being mainly inductive below the self resonant frequency and capacitive above it.

    You could probably design an amplifier with a gradually rising input impedance to match the loop impedance if it was purely inductive over the desired frequency range. But if you have a resonance in the middle of the frequency range, it becomes a lot harder to design an amplifier with a response that tracks the loop impedance.

    Loops made from thin wire (high value of self inductance) which are screened (adds additional capacitance) are a bad combination. This is one of the problems with the Pixel or Mobius loop antenna designs, that stop them performing as well as other much simpler designs.

    The general rule is to use thick conductors to minimise loop inductance and good balance so that you don't have to screen it.

    All of the above is a bit of a generalisation, but some further detailed notes on the subject can be found here.



    Regards,

    Martin - G8JNJ








     
  • KA7UKA7U
    Martin,
    The references are excellent although probably written with more education on the subject than I possess. Hi Hi  Anyway, these articles prompted me to make a crude SNR test across the KiwiSDR band spread. With the LNA disconnected from the antenna the waterfall is scanned for average noise floor, then the LNA is placed in line and the DC voltage supply is adjusted to peak the 60KHz WWVB time signal until the noise floor starts to rise, then the voltage is backed off just a bit. This adjusted DC voltage is 8vdc. The band spread is then inspected again looking at the average noise floor and subjectively observing the general SNR. This test might be made more determinate if I had a frequency agile signal source connected to a resistive load to try to measure the SNR across the band, but I have not gone to that level and there probably is a better way with the right equipment to measure such things.



    Ron
    KA7U
  • G8JNJG8JNJ
    edited February 2018
    Hi Ron,

    That looks pretty much as I'd expect it to.

    If you take a look at my KiWi (zoomed out to WF0)


    Switch between antenna 4 (LZ1AQ loop low input Z amp) and antenna 7 (G8JNJ loop with higher input Z amp). Both loops are the same size (made from LDF 4-50) and have more or less the same orientation. But you should see the MF signals are higher in level on antenna 4 and the HF signals are higher on antenna 7, the only difference is the input Z of the loop amplifiers.

    If you want to quickly check the frequency response / loop gain of your design, you could possibly use something with an commutating electric motor (not non-brushless) like a cordless electric drill, shaver, hair clippers, toothbrush or similar battery powered compact device placed near the loop as a wideband noise source. Avoid corded items as they will tend to have resonances that will not give a flat response.

    Regards,

    Martin - G8JNJ




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