A look at the MFJ-1790 Portable 10 Meter Vertical Antenna

[Moleculo]

Not too long ago, MFJ started marketing the MFJ-1790 10 meter vertical as a quick-install antenna that is perfect for portable operations. I contacted MFJ about obtaining one for an article and they sent me one to try out.

Here's everything that comes in the bag, laid out on the ground:

Here's a full length picture. The longest aluminum tubing are about 5 feet long, which doesn't make this antenna the smallest portable antenna available, but at least it's manageable.

Assembly of this antenna is pretty straight forward. This is the hub that attaches to the mast and supports both the vertical and horizontal elements.

Simply attach both the horizontal tubing and then the bottom section of vertical tubing. The ground plane radials are securing with the supplied hose clamps and the vertical is secured with a small machine screw.

Attach the top section of vertical tubing and then measure the overall length to set it at 11 feet, which is the length indicated by the manual.

Mount the antenna at least 3 feet off the ground to test for resonance:

I initially hooked up the MFJ Analyzer and couldn't find any frequency where the antenna would work, so I knew something was wrong. After taking the antenna down and unscrewing the hub from its base, I found this:

The MFJ-1790 claims to be an off-center fed vertical. As you can see, the matching section is inside the hub. The antenna I was shipped had the center conductor broken off during assembly. I quick solder job fixed the problem, but when I put it back together, the tight assembly made it easy to understand how something like this could happen.

Now that I had the antenna assembled properly, I got it back on the air and found the frequency where the antenna was initially happy:

Seeing that it was much too long, I shortened the top radiator and was ready to go within a few minutes:

I hooked up the MFJ-9410 10 meter SSB transceiver and was ready for some back yard QRP fun!

A couple of notes about this antenna: The tubing is pretty thin walled stuff and will get damaged easily if the antenna comes down and falls on a radial. In fact, the radials can be bent pretty easily right at the hub. The antenna is easy to install, but you should check out the matching section inside the hub before you take it to the field to play with. I wouldn't claim that the antenna works any better or worse than any other antenna of similar design, except that it's definitely not intended for permanent install due to the thin tubing. The antenna retails for about $70 and would be a good choice for working 10 meters at a campground, backyard or similar location where space is a premium.

 

[w9cll]

Nice piece of work here.

 

[Road Squawker]

what is that sitting on the radial?..... looks like some kind of spray can

 

[Moleculo]

what is that sitting on the radial?..... looks like some kind of spray can

LOL, optical illusion. It's a tiki torch in the background.

 

[Robb]

It first tuned up on the freeband correctly; and not on 10m?
LOL!

That is quite a difference in length, a few inches isn't a small mistake.

 

[mr_fx]

hmmm... not impressed, I would rather hang an a wire in a tree... it would likely perform a little better too

 

[Lil'Yeshua]

Would some DX Engineering rigid tubing hold up better strength wise?

 

[ghutch]

Take this little antenna to the beach.......put its mast right at or in the saltwater and you'll see it is a real performer. Lite weight is great for a portable. 10 meters and 12 ? Should be easy to tune.
I used a 4 radial recently at the beach....worked on 10-40 with my tuner......works well.

 

[Moleculo]

Would some DX Engineering rigid tubing hold up better strength wise?

Yes.

 

[Riverman]

Yes.

I like the simplicity of this antenna but just don’t get the configuration. It resembles a quarter wave 10 meter antenna but the vertical element is 132 inches rather than 99, and the radials are only 72. With the correct dimensions and the radials slanted downward, no matching unit would be needed and it would likely be more efficient. Wonder what the reasoning was behind this design?

 

[Rick330man]

I like the simplicity of this antenna but just don’t get the configuration. It resembles a quarter wave 10 meter antenna but the vertical element is 132 inches rather than 99, and the radials are only 72. With the correct dimensions and the radials slanted downward, no matching unit would be needed and it would likely be more efficient. Wonder what the reasoning was behind this design?

I am thinking like you. Take a rail mount, snip a 102" stainless steel whip down and run a counterpoise. No matching hub/coil/network; no risk of broken wires, etc.

 

[Captain Kilowatt]

The dimensions sound as if it was meant to be similar to a Carolina Windom antenna which is similar to a half wave dipole but with off centre feed with one side 1/3 wavelength long and the other 2/3 wavelengths. 132 inches is pretty much 2/3 and the 72 inch side is pretty much 1/3 of the overall length except this has a two section counterpoise instead of everything being inline.

 

[Riverman]

.

The dimensions sound as if it was meant to be similar to a Carolina Windom antenna which is similar to a half wave dipole but with off centre feed with one side 1/3 wavelength long and the other 2/3 wavelengths. 132 inches is pretty much 2/3 and the 72 inch side is pretty much 1/3 of the overall length except this has a two section counterpoise instead of everything being inline.

Sounds right.
What would be the intended benefit of that design over a simple quarter wave?

 

[Captain Kilowatt]

.

Sounds right.
What would be the intended benefit of that design over a simple quarter wave?

For use as a single band antenna I cannot think of any advantage. The whole idea of an off-centre dipole aka Windom antenna is to allow multi-band use without a tuner on even harmonically related bands. Some good info is here. https://www.onallbands.com/off-center-fed-ocf-antenna-multitalented-or-misunderstood/

 

[The DB]

I was asked in a PM about my thoughts on this antenna, I decided to post them here in case anyone else was interested.

As Captain Kilowatt noted, this antenna has similarities to the Carolina Windom, which is essentially an off center fed dipole. While such antennas clearly are intended for multi-banding, that is clearly not the case with this antenna. So, what is the purpose of using said techniques on this antenna?

A 1/4 wavelength vertical antenna over a perfect ground has a resonant feed point impedance of 36 ohms. Replace that perfect ground with four 1/4 wavelength radials and raise it far enough above the earth and the resonant feed point impedance drops to about 20 ohms. Remove two of the four radials and it drops even further.

Then factoring in one of the things that is known to happen with an off center fed dipole, moving the feed point from the center raises feed point impedance, so in this case they lengthen the vertical element and shorten the radials until the antennas resonant impedance comes back into line. This also gives them a longer vertical element on the antenna, which many people think is the most important part when it comes to antenna performance.

If they got the dimensions right, the balun in the case of this antenna should be a 1:1, and that is what it looks like with how little of it I can see in the manual. If it is a 1:1 balun, the only thing it is doing with this antenna design is helping prevent common mode currents from flowing back up the feed line, something that will be more prevalent than a design that uses full 1/4 wavelength radials.

Assuming it is far enough above the earth you can also run this antenna as a horizontal antenna, although I would think it was simpler to just put up a dipole or some other wire antenna which will work just as well or better, and be far easier to mount.

The one thing I find suspect in this antenna's manual is how close to the ground below you can mount this antenna. They talk about tuning at three feet and then raising it to mounting height. I would go higher than that as I think this antenna's feed point impedance will change with antenna height at low elevations, at least 10 feet up when tuning if you plan to mount it higher.


The DB