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Thread: RF Level Equations?

  1. #1
    The Fluid of Spock KD8TUT's Avatar
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    RF Level Equations?

    Hi guys,

    I've run into a niggling problem with my station. It goes something like this:

    I've got two Windoms about 35 ft apart. One (80 meter resonant) is used for transmit and receive. The second Windom is 40 meter resonant and inverted form the first- used for noise sensing and beamforming.

    System works great, can null out almost any noise which appears on both antennas.

    The problem is during transmit. Too much RF is coming into my second receiver, ADC 2, and is over driving it. I can compensate at some transmit levels with a step attenuator. I've also ordered an RF limiter for the second antenna which should solve the problem.

    What I'd like to know is what equation can I use to model the return RF on the second antenna if the gain of both antennas is known?

    Since I've never really done antenna modelling or setup a field day station I'd like to understand the problem fully even if I've already found a fix.
    --
    So there I was, totally naked. With only a rubber hose and a stuffed animal...

  2. #2
    The Fluid of Spock KD8TUT's Avatar
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    Tried an RF limiter that can cancel 10 watts of reflection from the secondary antenna. Did not work.

    Reconfigured the station yet again to make it work- but not how I want it to.

    Looks like I'll need to put a relay on the receive antenna to completely disconnect it during transmit- or make a minor modification to the board in the radio.
    --
    So there I was, totally naked. With only a rubber hose and a stuffed animal...

  3. #3
    Volcano Tamer PA5COR's Avatar
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    Running up to 800 watts here i have several antennas in use resonant on the frequencies.
    Including an amplified receive antenna, feeding the separate receiver in the FT 2000 - D where i put 2 anti parallel switched Schottky diodes to limit the output of the antenna amplifier when transmitting.
    The other antenna's are running through the Heathkit antenna switch that pulls every not used antenna port to ground, in effect detuning the antenna's which helps.

    Any antenna will pick up serious R.F. when withing 1/2 wavelength of the transmitting antenna, so i made up the next receive antenna
    The amplified antenna is a home brew probe antenna with a dual gate Mosfet ( BF 961) where both gates are coupled together and run very hard in class A ( 30 milli amps current as impedance converter, followed by a BFR 96 S amplifier and BFW 16A 1 watt 50 Ohm coax driver all run in class A, and can withstand 1000 watts at 5 meters away without damage....;) the actual probe is 4 x 2 Inches PCB double sided copper.
    "If the Republicans will stop telling lies about the Democrats, we will stop
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  4. #4
    Administrator N8YX's Avatar
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    On which frequencies are you transmitting when you see the overload occur?
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  5. #5
    The Fluid of Spock KD8TUT's Avatar
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    Quote Originally Posted by N8YX View Post
    On which frequencies are you transmitting when you see the overload occur?
    Pretty much all of them lol.

    I tested the return wattage from the secondary receive antenna (over 10 watts when amp is at 500 watts) and then checked my work with an EE in my local ARC..

    Based on the proximities involved I may need to isolate that antenna to ground during transmit.

    My initial reaction was to do some math but my friend Earl (N8SS) informed me that the measurement I did was better since the math would only be accurate in a controlled environment- these antennas are in a forest so the results may or may not be accurate.

    As a side note- this is when I'm thankful for our wide coverage repeater which allowed me to talk over the problem with Earl for an hour as I returned from a client site.

    Bottom line is that if I want to use adaptive predistortion on transmit, with phase cancellation on receive, there is no choice but to use ppt---->relay---->antenna---->ground. Unless I want to modify the radio internally or buy a revised PA.

    There may also be another issue- common mode on the secondary receive coax. I have a good line isolator to test that. So I'll eliminate that and then head to the ham store for a relay and a few doo dads.

    It's possible I may buy a revised PA, or purchase the board and build it myself. But in the short term if grounding the antennas resolves the problem I'm good with it.
    --
    So there I was, totally naked. With only a rubber hose and a stuffed animal...

  6. #6
    Administrator N8YX's Avatar
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    Consider both isolating the antenna from the other station apparatus and terminating it in its characteristic impedance upon entering transmit. Find an old Dow-Key relay and connect the antenna to the Common port, the receiver to the NC port and a screw-on 50-ohm dummy load (Diamond DL-30 or equivalent) on the NO port. Use an ARB-704 or similar switching controller to activate the relay on transmit. Most of the Dow-Key units have a set of leaf switches that operate in conjunction with the main armature so you can incorporate a TX delay/lockout circuit which won't allow the station PA to come up until after the Dow-Key is fully in the 'Transmit' (energized) mode.

    You won't be able to run QSK CW with such an arrangement so if that's a requirement I'd look into a high-power PIN diode isolator.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

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