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Originally, I built an 80m EFHW, 130.5 feet long cut for approximately 3.58Mhz. I attached a 49:1 UnUn at the feed point 13 feet above the ground with coax back to the shack. I did not use a counterpoise but tied the secondary of the UnUn transformer to shield of the coax. It worked very well, out to Australia on the West and out to Russia on the East. The antenna is shaped like an inverted V, both ends at 13-15 feet in the air, the apex is about 45 feet in the air. The Inverted V apex is raised to 30 degrees above horizontal. The SWR was reasonable on 40m, 1.9.

I decided to experiment and built the same inverted V shape with a wire 117 feet long with feed point connected to the same 49:1 UnUn. This time I am using a 13.5 foot wire for a counterpoise, no connection to coax shield, overall length is again 130.5 feet. The 80m dip in SWR now occurs at 4.0Mhz and not at the 3.58Mhz as expected. I doubled the length of the Counterpoise and there is no difference. The dip is still 4.0Mhz. I wanted to tune the SWR dip by changing the length of the Counterpoise. I doesn't look like I can. What am I missing????

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Originally, I built an 80m EFHW, 130.5 feet long cut for approximately 3.58Mhz. I attached a 49:1 UnUn at the feed point 13 feet above the ground with coax back to the shack. I did not use a counterpoise but tied the secondary of the UnUn transformer to shield of the coax.

Unless you used a common mode choke, the outside of the coax cable serves as (a long) counterpoise. As the shield is now part of the antenna, it picks up all the electrical noise in the vicinity and the noise floor is raised on 80m.

I decided to experiment and built the same inverted V shape with a wire 117 feet long with feed point connected to the same 49:1 UnUn. This time I am using a 13.5 foot wire for a counterpoise, no connection to coax shield, overall length is again 130.5 feet. The 80m dip in SWR now occurs at 4.0Mhz and not at the 3.58Mhz as expected.

For 3.58MHz the antenna (main wire) needs to be lambda/2 at 3.58MHz (about 130.6 feet) and the counterpoise 0.05 lambda (about 13 feet) long.

You actually don't need the extra counterpoise wire. Tie both primary and secondary of the UnUn to the shield of the coax cable and put a good common mode choke about 13 feet away from the transformer. This avoids common mode currents and helps reducing the noise level.

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    $\begingroup$ Yes, I do have a common mode choke on the coax to prevent the shield from being used as part of the antenna. I forgot to mention this. $\endgroup$
    – Ron K5TDF
    Commented Apr 23 at 13:09
  • $\begingroup$ By shortening the main wire to 117 feet, connecting to the 49:1 UnUn and connecting to the 13.5 foot counterpoise, I would have thought that this would tend to be and OCF dipole that could be tuned with the length of the 13.5 foot wire. Now that I see that the main resonant point is determined by the 117 foot length and does not respond to the doubling of the 13.5 foot wire to 27 feet, now I begin to question the use of the 49:1 UnUn. I basically $\endgroup$
    – Ron K5TDF
    Commented Apr 23 at 17:34
  • $\begingroup$ I basically have placed an impedance of 2450 ohms between the two lengths of wire. Would this tend to isolate the two wires forcing only the 117 foot wire to determine the resonant point at 4.0MHz instead of 3.58MHz? $\endgroup$
    – Ron K5TDF
    Commented Apr 23 at 17:42
  • $\begingroup$ Look at the EFHW as if it is a lambda/2 vertical monopole working against a modest ground. Making the monopole shorter will shift the resonance up. Improving grounding with more/longer radials will not change that. OCF-dipoles are fed at a larger offset (20...30%) from the end with a a low impedance balun (4:1). In order to cover all harmonic bands, the dipole is cut longer than neccessary and then shortened electrically on the lowest band with a capacitor inserted in the middle of the dipole. Because the antenna is unbalanced (high CMC) a a very good chocke at the feedpoint is recommended. $\endgroup$
    – Raonoke
    Commented Apr 26 at 6:40
  • $\begingroup$ gnarc.org/wp-content/uploads/K1RF-OCFD-Antennas-R2C.pdf $\endgroup$
    – Raonoke
    Commented Apr 26 at 6:42

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