Using a small QRP high impedance output TX with a 52ohm antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K9JMS, Jan 4, 2020.

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  1. K9JMS

    K9JMS Ham Member QRZ Page

    This question is related to part 15 AM broadcasting, but I know the experts are here and this same question could relate to any frequency in the ham bands. I have a 100 mw TX that transmits @ 1690 am. It comes with a 3 meter whip antenna. Per the manufacturer, the antenna output is at a high impedance. The could not tell me the exact impedance though. The rule says I can use any antenna 3 meters or less in physical length. I have a 160 meter Valor Pro-Am mobile antenna. I exchanged the factory whip for a longer one and it is now tuned for 1.69 mhz, It's input impedance is 52 ohm. What is the best way to match the TX to the antenna with minimal loss?
     
  2. K7MEM

    K7MEM Ham Member QRZ Page

    Well, it's common to use a L-Network tuner (L-Series/C-Parallel) when driving a high impedance antenna (End-Fed for example) from a 50 Ohm transmitter. Why not try turning that around. You should be able to construct a simple tuner to handle the mismatch.
     
    WB5YUZ likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The place to do the matching is at the transmitter in this case, not at the antenna.

    If you can copy and paste the schematic diagram for the little transmitter and it has components and supply voltage identified, it may be possible to calculate what is needed; and again, what is needed would be a matching circuit at the transmitter itself. The supplied 3m antenna must have been connected directly to the transmitter without any transmission line involved and the circuit is likely trying to match a load that has extremely low resistance and extremely high capacitive reactance. As soon as you move the load away from the source and add a transmission line, everything changes.
     
    WB5YUZ likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    A 3m (9.8ft) monopole fed against ground has a feed point impedance of 0.196 - j4357 at 1.69MHz. Assuming that the transmitter was designed to match that impedance (big assumption!), then here is what it takes to transform that Z to 50 Ohms:

    Shunt the input of the 50 Ohm antenna to its own ground with a 31.7nF (yes that is nano) capacitor. Put a series 434uH (yes, that is micro) inductor between the transmitter output terminal and the 50 Ohm antenna input. Connect the ground of the 50 Ohm antenna to the transmitter ground.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I might mention and forgot to (above):

    A very good 3m long mobile whip is unlikely to work any better than a directly-attached 3m long whip. Actually, a directly-attached 3m whip with no transmission line should outperform any kind of 3m long antenna you connect "remotely" at the end of any kind of transmission line.

    I haven't studied the regulations for license-free AM BC stuff in years, so I don't remember if the regs allow a great radial system under the whip; but if they do, simply placing the 100mW transmitter in a weatherproof enclosure and locating it outside with its 3m whip (something that will also last outdoors) and connecting lots of radials to its chassis or ground plane, spreading them around on the ground under the whip, should boost the ground wave signal quite a bit. On a clear enough frequency, I'd think it could be copied for several blocks, maybe farther.
     
  6. K1VW

    K1VW Premium Subscriber QRZ Page

    I seem to recall an opamp buffer is a device which converts a high impedance input to a low impedance output. I don’t have any experience with this, but it may be possible to use one with this power level and relatively low RF frequency.
     
  7. KH6AQ

    KH6AQ Ham Member QRZ Page

    What is the transmitter and do you have a link to the manual or schematic?

    I have a Talking House AM BC transmitter and it has both an internal autotuner and a 50 ohm output. The FCC allows a 3 meter antenna measured from ground to the tip of the antenna. Radials are allowed but elevating the antenna above ground, the ground lead counts towards the 3 meter length.
     
    N8TGQ likes this.
  8. WB5YUZ

    WB5YUZ Ham Member QRZ Page

     
  9. KH6AQ

    KH6AQ Ham Member QRZ Page

    Additional information for those not familiar with unlicensed AM BC operation:

    Maximum transmitter power is 100 mW DC input to the final amplifier.
    No feedline is allowed; the transmitter must be mounted at the antenna.
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd think the most effective way to increase range, then, would be to place the transmitter on the ground in a clear area and add a lot of radials.:)

    I'd also think the transmitters should be designed to drive a very high impedance, since Rr is <<1 Ohm but Xc is >>3000 Ohms.
     

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