ad: elecraft

MFJ-259C readings jumping all-over on skywire loop

Discussion in 'General Technical Questions and Answers' started by K9SPY, Dec 29, 2015.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: Left-2
ad: L-MFJ
ad: Subscribe
  1. K8JD

    K8JD Ham Member QRZ Page

    I can see where the night time site for KZDC could be a problem with only 920 W they are close at 2.7 miles. They share towers with KTSA at night.
    The daytime site is just over 13 miles away to the southwest of Kirby. Still a problem at 25000 W ! The daytime antenna's big lobe is just off Kirby.
     
  2. K8JD

    K8JD Ham Member QRZ Page

    When you consider the inverse cube law re distance vs field strength, even a fairly high power AM broadcaster doesn't have a very intense field more than a few miles from the transmitter.
    Maybe some one else has the interest to do the exact math here.
    KZDC has a night time field strength of about 350 MV/Meter at a mile, or is the current standard a kilometer ? And that is at the fundamental and should be a LOT weaker at the 3rd harmonic.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Anything's possible but I doubt it's AM BC doing this.

    If it is, a high pass filter between the antenna and the analyzer should eliminate it, and MFJ does sell one just for that purpose.
     
  4. W0BTU

    W0BTU Ham Member QRZ Page

    It could be something else, but the MFJ-259B and similar analyzers are notorious for nonsense readings from nearby broadcast stations.

    www.w8ji.com/mfj-259b_calibration.htm
    explains why:

    ... The bridge detectors are NOT frequency selective, and respond to anything from minor dc offsets through microwave signals. This causes inaccuracies if any voltage over a few millivolts appears across the antenna port. (This is also true for competing analyzers from other manufacturers.) There are multiple reasons why, at the time of design, these units were dc coupled with broadband detectors. ... The MFJ259 series RF power level is about 10 dBm, although this varies with the load impedance. Since the bridge depends on nulls, any external voltage will throw off readings. ...

    Because the detector is broadband and because it is dc coupled to the antenna, any external voltage across the antenna input port causes measurement errors. It is the accumulated voltage of multiple sources that is most important, not the strength of any individual signal. Because of that, large antennas should be tested at times when propagated signals in the range of the antenna's response are at minimum strength. ...
    AND from the MFJ-259C manual http://www.mfjenterprises.com/Downl...filename=MFJ-259C Manual Ver5.pdf&company=mfj :

    ... all handheld analyzers share certain limitations, and being aware of them will help you to achieve more meaningful results.
    1.4 Local Interference: Like most hand-held designs, the MFJ-259C uses a broadband directional coupler that is open to incoming signals across the radio spectrum. Most of the time, the unit's built in +5-dBm RF generator is powerful enough to override any interference caused by stray pickup. However, under some circumstances, a powerful nearby transmitter could inject enough RF energy through the antenna being tested to overload the coupler and disrupt readings. If overload occurs, measurements may become erratic or SWR readings inaccurately high. These occurrences are rare, but if it becomes a problem at your particular testing location, the MFJ-731 Tunable Analyzer Filter is especially designed to notch out unwanted signals with minimal impact on analyzer accuracy.​
     
    Last edited: Dec 30, 2015
    N0SYA likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Of course, and the problem isn't just with the MFJ units. RigExperts' AA-series which I have on my bench now does the same thing.

    However, I've used the 259 in the field for several years and have only seen this happen when I'm setting up for Field Day on a local hilltop shared with powerful commercial stations including FM BC. If I drive just a mile away, the problem disappears completely.

    I think an AM BC groundwave signal would need to be from a very nearby transmitter for this to occur; and when it does occur, the problem isn't intermittent as described above, it's pretty continuous.
     
  6. K4SAV

    K4SAV Ham Member QRZ Page

    My MFJ analyzer is useless on 160 because of a nearby AM station. The meter swings all over the place with modulation from the AM station. If the modulation stops I get a steady reading (steady wrong reading).

    Jerry, K4SAV
     
  7. K9SPY

    K9SPY Ham Member QRZ Page

    update: found a section of wire ripped back through pulley and some insulation shredded... my highest attachment point, and my most shrouded in branches and foliage attachment point!

    when I moved the mount and feedpoint to the tower the other day, the slack in the wire must have allowed the counterweight to suck dacron and insulator and everything through... then the wind storm chaffed the heck out of it...

    untangled all that and things were finally back to predictable!

    believe it or not the copper is actually intact

    thanks for the suggestions!

    K9SPY
     
  8. K8JD

    K8JD Ham Member QRZ Page

    Glad you found the problem there. I didn't think an AM transmitter farther than 2 miles away would do what you were seeing .
    Jerry's AM nemesis is only about a mile and a half from his antennas !
     
  9. K4SAV

    K4SAV Ham Member QRZ Page

    Well there is a station at about 2 miles from me but it is very low power. It's the one at 47 degrees and 1700 ft from my receiving antennas that does a number on my analyzer.

    Jerry, K4SAV
     
  10. K9SPY

    K9SPY Ham Member QRZ Page

    Correction: Thanks for some REALLY FANTASTIC technical insights!

    :)

    K9SPY
     

Share This Page