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The FXTenna Portable HF Antenna

Dave Land (KD5FX) on May 26, 2011
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The FXTenna (TM) Portable Antenna 
by Dave Land KD5FX

20 through 6 meters, QRP to full legal power, lightweight, easy to carry

I've been looking for an inexpensive portable HF antenna for my new (to me) ICOM IC-706 radio. I looked at several models but most were rather expensive. I found several designs on the web to build your own but most involved coils and covered the lower bands. I was more interested in 20 meters and above since the HF bands are coming back now that the solar cycle has finally rolled back around (this is my 3rd solar cycle!). 

I did find one design that used the 10 ft MFJ-1954 telescoping antenna so when I went to look that up on the MFJ website I noticed they also have a 12 ft model for just a little more! Hmmm, what bands could I cover with a 12 ft antenna? So, I did the math and discovered that it would cover 15 through 10 meters! Well, it could certainly cover 6 and 2 meters too! Just shorten it up! But I'd like to cover 20 through 10, what can I do? After some more math I realized that a couple of 3 ft 3/8" aluminum rods would make it cover down to 20 meters. I could use one 3 ft rod to cover 17 meters and two for 20 meters.

I was able to find the aluminum rods for a little more than $5 each at Lowe's and already had a 3/8" 24 thread die so I could thread the rods to work with my 3/8-24 mobile antenna mount. I got that mount at a local truck stop, in their CB radio supply area. Lots of various 3/8-24 mounts there, I chose one that will work in the yard or with longer bolts I can mount it on top of my SUV and go mobile HF with it too but not with this antenna! MFJ says not to use this telescopic antenna while moving, it will break! I plan to use a 'hamstick' type HF antenna on the truck.

 I also needed some 3/8-24 couplers to hold the two 3 ft rods together and mount the 12 ft whip on top. I was able to find those at the CB shop and later I discovered that you can use 1/8" NPT pipe couplers, the thread is a little different but will work according to what I've read online. I used the Stainless Steel (SS) ones I found at the CB shop, they were cheap. 

This antenna covers the whole band with a low SWR and best of all it is very efficient since there are no coils involved! The optimum length for your ground radials are shown in the table below, it will work OK with something shorter, I'm using eight 114" radials.

Here's an equipment list:

EXTRA-LONG TELESCOPIC ANTENNA, 12 FT (758-1196)  MFJ-1956 Price: $34.953/8-24 mirror mount for CB antennas from the local truck stop (about $10)
3/8-24 SS couplers from the local truck stop (about $4 each, you need 2)
3 ft 3/8" aluminum rod from Lowe's (about $5 each, you need 2)
3 ft 1/4" or 3/8" steel rod from Lowe's for a ground stake (about $5) Cut it down to 24", 3 ft is just too long!
3/8"-24 thread die to cut the threads on each end of the aluminum rods
4 to 8 ground radial wires, I used some 114" wires left over from another antenna, longer is better if you have the wire, see the table below
3 - 8 to 10 ft long 1/8" nylon ropes or heavy duty fishing line to use for guys, I attached them with a small tie wrap
1 large nail to use for the end of each guy rope and for each radial wire, I use 16d size, they are about 4" long

Here's a table of the dimensions you need for each band.
Note that for 15 to 6 meters you use only the 12 ft telescoping antenna from MFJ and you shorted it the 'amount to shorten' 
For 17 meters you use one 3 ft 3/8" rod plus the MFJ antenna shortened by 1.53 ft
For 20 meters you use two 3 ft 3/8" rods plus the MFJ antenna shortened by 0.85 ft

Band Freq Length (Ft) Extension to 12 ft Amount to shorten (ft) Radial length req'd
6 Mtrs 50.1 4.87 - 7.13 4.79
10 Mtrs 28.3 8.62 - 3.38 8.48
12 Mtrs 24.93 9.79 - 2.21 9.63
15 Mtrs 21.2 11.51 - 0.49 11.32
17 Mtrs 18.11 13.47 3 ft  1.53 13.25
20 Mtrs 14.225 17.15 6 ft 0.85 16.87

For 17 and 20 meters you'll need to guy the antenna with some small nylon rope or fishing wire. I use large nails to stake the end of the guy ropes and ground radials in the grass/dirt. Might need to do this even for 15 meters and above if it's windy.

Here's some pictures:

First off, here's the MFJ 12 ft telescoping antenna collapsed all the way at the top of this picture and one of the 3 ft 3/8" rods below it
Next is the 3/8-24 mirror mount bolted to a 1/4" steel rod (still with some dirt on it)
The grey wire is the 114" radial wires, I use 8, longer is better, see the table below
The white small nylon rope is used to guy the antenna when it's windy, tie it on with a wire tie
Last is the 16d nails I use to secure the ends of the radial wires and the guy ropes.

Now here's the antenna setup for 15 meters, it was rather windy, you can see it leaning a bit.

And here's a close up of the CB antenna mirror mount. It's clamped to a 24" long 1/4" rod pushed down in the dirt.
Note how the radial wires are connected to the mount.

Below is the antenna setup for 17 meters, one 3 ft rod installed and the 3 nylon guy ropes

Member Comments:
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The FXTenna Portable HF Antenna  
by W6USC on May 26, 2011 Mail this to a friend!
Nice job ill give it a try 73s Bruno
RE: The FXTenna Portable HF Antenna  
by N4JTE on May 26, 2011 Mail this to a friend!
Ain't going near this one other than to say always nervous when someone names an antenna after themselves. Especialy this one.
Tnx for sharing,
RE: The FXTenna Portable HF Antenna  
by AD6KA on May 27, 2011 Mail this to a friend!
""and best of all it is very efficient since there are no coils involved!""

Hardly.Not with a mere 8 ten foot ground radials!
(Yes, I realize it's a portable antenna, but don't call it "efficient" because it doesn't have a coil)

""This antenna covers the whole band with a low SWR""

Yet ANOTHER indicator that it is NOT efficient at all.
That wideband low SWR is caused by ground losses due to a poor radial field.

"Legal Limit" On an MFJ telescoping mast? Did you actually try this yourself??

You've got a great start on a good, portable antenna.
I applaud your "homebrew, find the parts & go build it" attitude. It's a 1/4 vertical plain and simple. If you want to make it more "efficient" do some more studying of antenna theory and what "antenna effciency" really means.

73, Ken AD6KA
RE: The FXTenna Portable HF Antenna  
by KD5FX on May 28, 2011 Mail this to a friend!
Well, no, I don't have any more than 100 watts so I haven't tested it at 1.5KW, you could say that's an exaggeration. I've read extensively on antennas and antenna theory, I know to be very efficient it needs lots of radials but for a portable antenna how do you do that? Sure, it would be better to use tuned radials for each band and more of them but that is always the compromise that must be made with a portable. I used what I already had. If one is building this from scratch I'd recommend more and longer!
RE: The FXTenna Portable HF Antenna  
by AD6KA on May 29, 2011 Mail this to a friend!
"I know to be very efficient it needs lots of radials but for a portable antenna how do you do that?"

It's a PITA, that's true. But really, 20 radials will only take a few more minutes to lay down than 8.

"Sure, it would be better to use tuned radials for each band and more of them"

No, you only need tuned radials for an above ground atennna. Laying on the ground they are capacitivky coupked to the ground and detuned.

Sorry if I sounded harsh before, OM. You have a built a clever spiffy homebrew antenna that looks (and probably performs) at least as well as similar ones you see for sale. And saved a lot of money doing it yourself and carefully shopping for inexpensive bu "works great" parts. Give youself a pat on the back. TONS of hams these days can't solder a PL-259.
73, Ken AD6KA, ex 5R8GQ
The FXTenna Portable HF Antenna  
by VE2ITZ on May 29, 2011 Mail this to a friend!
It sound to me these days there is as many hams who love to have a condescending attitude towards other hams as there are those who do not know how to solder.

Might be some sort of power trip and make them feel important. Nevertheless, I would like to congratulate the author of this article for showing us his efforts in building his own portable antenna; even if it is not as efficient and is not full power limit as we would like it to be.

RE: The FXTenna Portable HF Antenna  
by N4JTE on May 29, 2011 Mail this to a friend!
Hope I am not in that category, but I do have a concern for antenna ideas that really are not good internet info due to intrinsic design deficencies, of course my thanks to the author for sharing his idea, No power trip intended, I am just coming from a fundamental knowledge of antenna physics and this idea is missing some really basic stuff.
RE: The FXTenna Portable HF Antenna  
by K9EZ on May 30, 2011 Mail this to a friend!
Then Bob, why dont you offer constructive ideas? Your first one was because it was "named" after him. If we have information that will make it better, why not share it. Else it is a power trip to say you know more, yet not help a fellow ham. Remember Elmering? How about mentoring?

Heck I use just about anything to get on the air, including using less than **GASP** 12 radials. And yes I know about grounding as I was Director of Engineering for a 50kW nine tower AM array.

Keep up the good work FX!
RE: The FXTenna Portable HF Antenna  
by N4JTE on May 30, 2011 Mail this to a friend!
Good point, that's why we have the very active Elmer's forum. I was hesitant to jump in because the design flaws seemed obvious to most of us and I did not want to pile on. The author skipped the Elmers forum learning process and went right to the "Article" section and attached his callsign to it, big jump, hi.
Simply put, the majority of the article dealt with how to make a 12 ft pole taller, and the multi band, non matched to anything design that is showing a low SWR because of all the mismatched conditions.
The radial lengths are well described but doing nothing as far as radiation resistance to speak of.
The whole point of any mono or multi band antenna, especally shortened verticals or too long verticals, is a well designed matching feed system. For portable use just build a 20 meter dipole throw it over a tree and feed with some ladderline for higher bands, I guarantee you it will work a LOT better than this idea and probaly a lot simpler to carry on a off road trip.
There is a very good reason multiband antennas have traps and loading coils.
Lots of great books out there to learn the basics but be aware of designs like this one that inspired you from the internet, it is full of crap info that often results in poorly designed attempts supposidly able to defeat the laws of physics.
Do some reading, keep building and please do not misinterpret my negative comments on the design as an attack on you.
Your article is well documented and presented but the old saying comes to mind; Putting lipstick on a pig.
Best regards,
RE: The FXTenna Portable HF Antenna  
by KD5FX on May 31, 2011 Mail this to a friend!
Bob, Did you understand it to be one length for all bands? If so, that is not what I meant. The idea is you re-adjust it for each band making it resonant on that band using the table shown, radiator and radials both should be tuned. Of course, a little tweek is required from what is shown in the table, those are a good starting point.
I guess I should make that more clear in the article.
73, Dave

The FXTenna Portable HF Antenna  
by AD5VM on May 31, 2011 Mail this to a friend!
Oh man... I commend you on all your hard work but if you look really closely in the last MFJ catalog, you will find they have a new whip MFJ-1979 that is sixteen foot, seven inches long, good for 20 meters by itself and it's made of stainless steel!! And they've addressed the biggest complaint about the old antennas being weak at the base, this one has a solid brass base that's one inch diameter and three inches long. It's got the standard 3/8 x 24 threads. It collapses down to less than two feet... Again this one is stainless!! not chrome plated brass like all the others. I just received mine the other day and it is just as described. It cost $72 shipped.
I built a super buddistick type antenna. I used a six foot tall 150lb capacity tripod used for PA speakers, a super HD antenna mount by Wilson with massive spring a 54 inch long hustler mast(model MO-3) homebrew 40 and 80 meter loading coils on 2 inch PVC (12 gauge THHN solid insulated wire) (15 turns for 40m and 47 turns for 80m) and then the 16.6 foot whip on top of that. So the feedpoint is six foot up, two elevated tuned radials each for 20/40/80 meters and the antenna is 28 foot high!! It's a little extreme for portable but with an MFJ-9475, nobody believes I'm QRP portable with a loaded whip antenna hihi...
RE: The FXTenna Portable HF Antenna  
by KD5FX on May 31, 2011 Mail this to a friend!
Wow! I did not see that! Must be brand new! Anybody want to buy a 12 ft MFJ whip?????
73, Dave
The FXTenna Portable HF Antenna  
by KQ0C on June 2, 2011 Mail this to a friend!
Just a few thoughts...

With a 16 or 18 foot fiberglass crappie pole (available at fishing stores) you can easily tape wire up to make similar portable antennas. An even longer fibreglass pole lets you elevate the feed point (see below). I use a 20 foot pole for back packing. Cut the antenna for 20 and roll up the wires to marks to retune it for higher bands.

If you reverse the CB antenna mount you can clamp your antenna into a pipe or stake you drive into the ground. Lightweight fiberglass snow marker sticks work fine as the stake. That will let you elevate the feed point and elevate your radials, which will probably double the efficiency of the antenna vs a modest radial field on the grass.

If you go with a longer radiator than 1/4 WL and with shorter elevated radials you can still tune the antenna to 50 ohms but have greater low angle gain. For instance a 22 foot vertical element with 10 foot descending elevated radials will give you quite a bit of performance at DX angles vs a 1/4 wave ground plane antenna.

RE: The FXTenna Portable HF Antenna  
by KC2UGV on June 3, 2011 Mail this to a friend!
To be fair, there was an apology for coming across as overly harsh...
The FXTenna Portable HF Antenna  
by OLLIEOXEN27 on June 4, 2011 Mail this to a friend!
Half wave wires hung from a tree branch solves all the coil, ground loss, radial problems.

Going mobile? Just use hamstick clones - about $15-$20 apiece.
RE: The FXTenna Portable HF Antenna  
by K5OX on June 5, 2011 Mail this to a friend!
Sometimes you can't find an answer in any books. I had posted here for some info on foil lined roofing. I figured it would shield my signal from the outside world. I finally stopped wondering and asking and just tried an antenna in this foil lined attic to see what would happen.

I had a Par 20 meter End Fed and strung it up as high as I could get it. I measured the SWR with a MFJ 829B analyzer and it was resonant and matched at 14.150. Per Par's direction. Added a stub of about 2 inches to bring the results to 1.1:1 SWR at 14.050, just about exactly what their own plot said it would be.

And I forgot to mention. It works great, both for short and long haul. It is maybe 20 to 25 feet high so low angle stuff is not great but I still work lots of DX with it.

The foil roofing liner is common in the high humidity gulf areas. And this type of roofing will lower your utility bills. But I've yet to get an explanation how the signals get in and out?

Anyone else with such a roof?

And to close I would like to say, The investment In at 829b type analyzer will save undue wear on my final transistors, and teach me a little about antennas.
The FXTenna Portable HF Antenna  
by W2CSH on June 6, 2011 Mail this to a friend!
Nice job Dave
Thanks for sharing. Just remember this is where no good deed goes unpunished. Those with ambition and curiosity do things, those without, write disparaging comments to your article.
RE: The FXTenna Portable HF Antenna  
by AA8GK on June 18, 2011 Mail this to a friend!
"this is e-ham and no good deed goes unpunished." Ain't that the truth? Whenever someone posts a good article like this one, I count how many replies until the flaming begins. Sadly, it's usually one or two.

The FXTenna Portable HF Antenna  
by AD7II on June 24, 2011 Mail this to a friend!
You've made yourself a fine antenna. Don't listen to the wire salesmen when it comes to radials. One works pretty well, it's sometimes called a counterpoise. With four or more you're in hog heaven. Eight is at the point of diminishing returns. The requirement for broadcast stations to use lots is so the takeoff angle will be predictable, not for efficiency. A quarter wave vertical is inherently broad band just like a half wave doublet. The shortie SteppIR controller for example has just one setting for 20 meters in the ham mode, although it will track you like a puppy in the general coverage mode.
RE: The FXTenna Portable HF Antenna  
by KD5FX on June 26, 2011 Mail this to a friend!
An UPDATE on this antenna. We used it for Field Day this weekend. Set it up for 15 meters to begin with and worked them left and right! Then changed to 10 meters and later 6 meters, it worked very well on all 3 bands. There were other stations and other antennas using the lower bands so we couldn't test it there.
73, KD5FX
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