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I am looking for a smallish omni-directional 10 meter antenna for installation in my attic. I notice that MFJ sells one called MFJ-1790. They call it an off-center fed vertical, and it has two horizontal radials. The length of the vertical element is 11 feet which is about about 5/16 wavelength at 28.4 MHz. So it is slightly longer than a 1/4 wavelength vertical.

https://mfjenterprises.com/products/mfj-1790

I was thinking of modeling it, and maybe trying to build one using wire, but I don't know how to feed it. There was a review of one of these, and from that you can see use of a toroidal choke at the base but nothing beyond that (the picture was to show a fatal assembly error lol).

https://www.worldwidedx.com/threads/a-look-at-the-mfj-1790-portable-10-meter-vertical-antenna.162599/

I can't find much on off-center fed verticals. Any idea if this a known design, and how you would feed it?

By the way, I am currently using an outdoor "stealth" ground mounted 1/4 vertical wire with ground radials, quite near to my house. It works OK, but I am wondering if I can get something elevated (about 18-20 feet) I will get better performance. The alternative it to build a 1/4 wavelength ground plane which is a lot easier, but maybe less performant.

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I'll answer your question with the technicalities, and then I'll make some suggestions. First, in a 50 Ohm vertical 1/4 wave antenna, as you might know, your radiator and your radials are each going to be 1/4 wavelength long, and your radials are going to be "drooping" approximately 135 degrees from the vertical element. These two factors, e.g., the lengths of the elements, and the separation between the radiator and the ground-plane elements are what give you a 50 Ohm impedance.

Since you want to place an antenna in a location where you likely cannot droop your radials, but rather have them at 90 degrees from your vertical radiator, that will naturally change the impedance at the feed-point to approximately 37 Ohms, and cause your VSWR to be 1.5:1 at its very best. To counter that, you can lengthen your radiator and shorten your radials (or vice-versa, in equal amounts; keeping the total length of the radiator to radial length a half-wave length). And you now have an off-center-fed antenna. If you have the floor space, you should have at least 3, if not 4 radials equally spaced in degrees of separation, and this will largely be determined by the space you have in your attic. You can easily build this antenna yourself, just make sure you use insulated wire and especially insulate the ends of each wire well, because these are your high voltage points on the antenna when you are transmitting.

As one alternative to this, you could just build an antenna out of wire with MFJs dimensions, and staple the vertical wire across the ridge board; the little section will make very little difference to VSWR, and if you feel it does, you can trim the wire back an inch at a time to see if VSWR improves.

The other alternative is to make a 1/2 wave end-fed antenna out of a wire that is color coordinated to your building, that you can discretely feed at ground-level, and suspend with a thin string, or monofilament, from a point on your building or in a nearby tree. I've done the calculations in the past, and you simply need a ~2uH coil, in series with the center conductor of the coax and the radiator, and a ~16pF capacitor going from the radiator side of the coil, to the shield side of the coax connector. If you make the coil out of bare wire, and make it ~2.2uH, you can move your tap point around the coil at your "center" frequency of the antenna until you get the lowest VSWR (or make the coil so you can compress it a bit, or expand it a bit to change the inductance). Here's an online coil designing calculator: https://www.66pacific.com/calculators/coil-inductance-calculator.aspx With regard to the capacitor, you didn't indicate how much power you are running, but I would use a cap with at least 600V rating; you'll be good for 100 watts then.

If you want to understand how to make a lumped solution for impedance matching purposes, use this calculator: https://leleivre.com/rf_lcmatch.html The Zs (Rs+jXs) is going to be set to Rs = 50, and jXs = 0 for the Zl (Rl+jXl), the Rl will be between 2450 and 3000, and jXl will also be 0. Set your center frequency, and calculate, and you will see the values needed with the circuit connections in the diagrams below. As an alternative to a lumped solution, you could buy or build a 49:1 UnUn, but you do want to buy, or build one out of ferrite type 52, or type 61 material; from what I've read from folks who have 49:1 UnUns for their end-fed 1/2 wave antennas, type 43 isn't as effective as a transformer material above a certain frequency.

Could you build an end-fed 1/2 wave antenna and bend it over in your attic? Yes, and it might be just a tiny bit better in efficiency than a 1/4 wave ground-plane antenna.

Finally, with any antenna, you should always choke the coax either at the feed-point, or a strategic distance from the feed-point if your design calls for using the coax as part of your antenna. In the designs I've described, you should choke the coax as close to the feed point as physically possible; this can be done by coiling the coax 5-6 turns in 9"-10" diameter, and zip-tying, or taping it into that shape, and/or use ferrite. There is plenty already written about this in other places so I'm not going to say anything else about that subject, except it is important, especially in the off-center-fed design, and the 1/2 wave end-fed antenna with the 49:1 UnUn.

One comment on your ground-mounted 1/4 wave antenna, is that any time you ground-mount a 1/4 wave antenna, you have a lot of ground loss, and that loss is improved (lessened), by simply going to a ground-mounted 1/2 wave antenna.

I hope this helps.

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    $\begingroup$ Thanks for your detailed answer – it was very helpful. I wrote a detailed response, but this site will not allow it due to length. The short response is that I now understand why that MFJ antenna is called “off-center”. About the vertical EFHW: my current outdoor antenna is a 10/15/20 “fan” vertical. I built a loading coil that I switch in on the 20m element to get 40. I may try your suggestion and use that same element to get an alternative 10m antenna. I could isolate the radial field for this configuration but don’t think I should have to. $\endgroup$
    – gschro
    Nov 30, 2023 at 17:20
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    $\begingroup$ Very good. I am a fan of fan configurations for applications that can't have multiple antennas, or the weight of traps and the narrow band width they create, or mechanical switching between elements. I'm sure you found that the 40M coil makes the bandwidth quite narrow, but it's better than not being able to get to 40M at all. And as you might imagine, there's no such thing as a fan EWHF, because EF 1/2 wave antennas are not fed at the lowest impedance point, that being a current loop, like a 1/4 wave ground plane or its cousin, the center fed dipole. $\endgroup$ Nov 30, 2023 at 23:22
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    $\begingroup$ Yes, bandwidth when on on 40m is low, and the radiation resistance is low, but it is still very usable. I think the take-off angle is higher, but that is OK because one reason for having that configuration is contacting stations in my 20m skip zone. For the coil, I tuned it by starting with a few extra turns, and then backing them out until it I got resonance mid-band. Regarding the EFHW, from what I have read, mica capacitors are a good choice and it looks like they are readily available rated at 1000V (I run 100W). $\endgroup$
    – gschro
    Dec 1, 2023 at 0:57

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