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I have an EFHW 40-10 in inverted L configuration on a fiberglass mast. I'd like to put an aluminum push-up mast nearby for a 6 meter antenna.

How close can I get to the EFHW with the push-up mast?

The ~67 ft. EFHW extends from 3 feet up to 46 feet on the fiberglass mast, and then over to another mast for the horizontal section. The most convenient location for the aluminum mast is 6 feet away from the fiberglass mast, on a right angle to the horizontal portion of the EFHW (think "L", where the horizontal portion of the EFHW extends due north). The aluminum mast extends fully to between 20 and 25 feet.

Update: Took SWR readings after work. Green is without the mast, orange with the mast.

enter image description here

Another update: Moved the mast 12 feet away, this time due south instead of east.

enter image description here

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    $\begingroup$ Approximately what distance do you mean by "nearby", how high on the fiberglass mast is the EFHW, and how tall will the aluminum mast be? Can you draw a sketch that includes the EFHW? $\endgroup$
    – Mike Waters ♦
    Oct 28, 2020 at 23:29
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    $\begingroup$ Thank you, Uncle Martin! ;-) I took the liberty of moving your comment to your question where it belongs. But with these details, someone might model it for you. $\endgroup$
    – Mike Waters ♦
    Oct 29, 2020 at 18:01

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Sounds like a small-lot problem. Nobody with personal experience with a 40m - 10m end-fed half-wave antenna has responded, so I'll chime in. I don't like unanswered questions ;)

If you bring a metal mast "near" your EFHW, the two antennas will interact. (Some would say that they won't be two antennas, they will be a single antenna with two ports.) I can think of two effects that you might notice: impedance changes and antenna radiation patterns. The impedance changes can change the SWR of one or both antennas.

So how close is too close? That's difficult for me to say without very specific details of your setup. It would be possible, but difficult, to model the antennas using antenna modeling software such as 4NEC2 or EZNEC. First we'd want to start with a 3D model of both antennas, their coax, and other nearby conductors. That would give us a good idea of the radiation pattern and impedance changes. Then if we knew details about your antenna tuner and your radio, we could estimate if the SWR would change enough to prevent a usable match or not.

Sorry, I'm not trying to be a smart aleck, it's just that I have more experience with modeling antennas than using EFHW antennas. Personally if I were in your shoes I would just try it, and make careful impedance measurements on both antennas with a good antenna analyzer before and after. If the impedance of either antenna system at the radio end of the coax changes enough to make a significant change in the SWR of either antenna, then I'd say that the aluminum mast is too close.

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    $\begingroup$ Thanks for the response! A rough test transmitting from the EFHW with the mast in place, at 100 watts, showed no significant changes in SWR at various HF frequencies. Out of my own curiosity, how will the low power from an antenna analyzer induce interaction with the mast? $\endgroup$
    – K8KV
    Oct 29, 2020 at 16:58
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    $\begingroup$ I'm glad I could help! The low power output from an antenna analyzer induces interaction with the mast exactly the same way a higher-power transmission from your regular rig would. The power is lower of course, but the SWR should be the same. If the SWR you measure with the analyzer is different from the SWR you measure with the rig, then one meter may be more accurate than the other. Or maybe the analyzer is being affected by interference; a strong nearby signal in the analyzer's passband could easily "compete" with the weak signal put out by the analyzer. $\endgroup$
    – rclocher3
    Oct 29, 2020 at 17:14
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    $\begingroup$ By the way I would have thought that 6' (1.8 m) would be way too close. That shows how much I know. If the SWR is only moving by a couple tenths at most in the usable ham bands, then I'd happily use that setup. Thanks for sharing the SWR scans! $\endgroup$
    – rclocher3
    Oct 30, 2020 at 0:39

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