Static charge on my EFHW?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0EP, May 24, 2019.

ad: L-HROutlet
ad: l-rl
ad: abrind-2
ad: Subscribe
ad: Left-2
ad: Left-3
ad: QSOToday-1
ad: L-MFJ
  1. KE0EP

    KE0EP XML Subscriber QRZ Page

    I'm assuming static is to blame for the jump in reflected power following several seconds of carrier transmission. It slowly creeps up and then jumps pretty high. The EFHW has the transformer, dedicated ground rod and CMC at the shack entry panel (also grounded). What can I do to bleed-off the charge? Will a 1m resistor do the trick?
  2. AA5CT

    AA5CT Ham Member QRZ Page

    re: "I'm assuming static is to blame for the jump in reflected power following several seconds of carrier transmission. It slowly creeps up and then jumps pretty high."​

    You're reaching the Curie point on your magnetics. The Curie point is where a magnet or magnetic material basically goes inert magnetically.

    The little ferrite core gets so hot - it LOSES its magnetic properties!

    You can try a small fan on the transformer/ferrite core, that's the only real cure for the power level you want to run.

    I ran into this situation a few years back with a loop design I was playing with - I went outside and checked the ferrites and they were HOT. I would watch my SWR meter and it too climbed and climbed ... went outside and the ferrite was very hot to the touch, so hot, I could not touch it.
    Last edited: May 24, 2019
    WG7X and AG5DB like this.
  3. WR2E

    WR2E Ham Member QRZ Page

    I've had that happen with some 'too small' baluns I was messing with.

    Not only ferrite gets hot either... a vertical antenna I built exhibited exactly the same phenom.

    Part of the matching network was a "plumber's special" sleeve capacitor. The type of plastic tube I used got so hot the plastic was melting out the bottom of the sleeve. At 50 Watts!

    Back to the drawing board.

    Don't touch it!
    AA5CT likes this.
  4. KW4TI

    KW4TI Ham Member QRZ Page

    A EFHW transformer should have at least one FT240-43 size toroid in it. Preferably two stacked. The losses can be up to 1.5 dB. This is a lot of power, especially if you are transmitting at legal limit. Better winding patterns can reduce this loss, but it's always going to be relatively high as the turns ratio is high.
  5. AA5CT

    AA5CT Ham Member QRZ Page

    I'll bet the losses get higher than that; 3 dB loss with 100W means the cores dissapate 50 Watts, and two 2.5 inch (thereabout) cores will get well warm with 50 Watts dissipated but still work.

    And 50 W SSB (peak) is more like 15 W to 20 W average dissipation.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Who made the EFHW and what kind of matching network does it have?

    How much power were you using to transmit?

    With the EFHW designs I've seen and also used, there wouldn't be any need to bleed off static, as the antenna is at DC ground potential, same as the outer conductor of the coax.
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Static charge does not result from transmitting power. Your static assumption is a red herring. As others have said, core saturation in your chokes or feed point transformer are more likely culprits and what you describe fits the description of typical core saturation.

    It could be the end fed matching transformer itself or it could be the common mode choke near the shack if it doesn't have enough choking impedance for the frequencies in use and there happens to be a lot of common mode RF on your feed line. I'd test by temporarily removing the common mode choke near your shack to see if the symptoms change. If the trouble goes away then replace it with a choke with substantially more choking impedance on the bands you use and or one with larger cores or stacked cores for better power handling.

    The late G3TXQ's chokes page can give you an idea of what choke designs and core materials might work better from a choking impedance standpoint:
    WG7X likes this.
  8. KE0EP

    KE0EP XML Subscriber QRZ Page

    Here's the MyAntenna products I'm using. Problem seems to be worse with the tuner in line. Why would that be? Feedline is 50' LMR-400. Power 100w or less.

    CMC-130S-3K.JPG EFHW-8010-1K.jpg
  9. AI3V

    AI3V Ham Member QRZ Page

    No, "common mode" would show a instant vswr, that will change when you touch the coax.

    Its the ferrite heating up.

    The high loss is why no sellers of this type antenna will bring one to a antenna measuring party.

    AG5CK likes this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Contact Danny at MyAntenna...he'll likely know the answer if you describe the problem fully and accurately.

Share This Page