Author Topic: MFJ-17758 80/40 Meter Dual-Band Dipole  (Read 11947 times)

Offline Alan Georges

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MFJ-17758 80/40 Meter Dual-Band Dipole
« on: July 27, 2013, 07:44:08 PM »
Imagine if you scratch-built a dual-band trap dipole out of commonly available hardware store parts, then built a little better one for a friend, then built another for another friend, refining at each step but not over-engineering things.  The result would still not be easy to work with, but once up it’s sturdy and functional.  That’s about what we’ve got here.  The antenna comes all assembled and in four coiled-up bundles of wire between the trap coils and center feedpoint.  Ninety feet of copperweld wire with various doohickeys on it is no damn joke when it tries to tangle on you, so be careful.  (This is emphatically not go-bag stuff-able.)  When you un-clip each loop of wire, do it outside and be ready to stretch it out, and hang onto it while you carefully unwind it.  It does have a really slick feature in that you don’t cut wire to adjust the operating frequencies.  Instead, you loosen and slide around these little aluminum blocks to take up or add extra length to each dipole leg, doubling up (or even tripling for the 40 meter part) the wire on that segment.  The instructions sheet has equations for how much the frequency shifts per foot you change things, and considering that it takes about 20 minutes to pull things down, adjust, and re-hang before making another SWR check (and I had to do this about four times to get everything tuned), it’s worth the time to do the calculations and measurements before adjusting.  BUT the slick part is that if you over-do an adjustment, the wire’s all still there and you just un-do things a little.  Clumsy, but very functional.

I had some ambitions to rig this antenna up for NVIS (inspired by this link http://www.hamuniverse.com/supernvis.html), but didn’t quite have room in the backyard for the full stretched-out length plus the end-poles and their guy lines.  So instead I rigged it as an inverted V with the center at 20 feet, because that’s the standard length of 2” PVC pipe and that’s what I had on hand, and ran three 22-gauge wires underneath, borrowing the reflector concept from that NVIS design page.

Back to the tuning, here’re the final SWR plots for this thing on 40 & 80 meters.  First on 40:


And on 80 meters:


To give this some perspective, the frequency axes of the two plots cover the General class phone range in each band.  For 40 meters, the entire range has good coverage.  It could stand being shifted a bit lower, but 2.2 SWR at the worst edge is acceptable.  If I ever get into CW, the entire 40 meter band can probably be accommodated by re-adjusting the antenna for slightly lower frequencies by lengthening it by a foot or two.  For 80 meters, the center 2/3 of the phone range is nicely covered.  I’d heard that trap antennas had narrow working bandwidths, and this confirms it.  Still, it is nicely centered and very useable.  Switching on the automatic tuner on the transceiver tamed the SWR down to a transmitter-friendly 1.1 across most of the range, getting as high as 2 near the edges of the useable part on 80 meters.  We are in business!

As late afternoon faded into early evening last Sunday, transmitting with 100 Watts from my near-Gulfport MS location I was able to easily make several contacts from the Houston area, and as far north as Birmingham.  The next evening I was able to talk to another ham in central Florida.  As expected, there wasn’t a lot of foreign traffic picked up by this arrangement.  Heard one guy in the Dominican Republic have a contact with somebody in the U.S., but the Dominican end was barely understandable.  So the bottom line is that this 40/80 NVIS rig gives good regional performance, rejects most signals outside of about 500 miles, adjusts closely enough to 50 ohms on both bands that it could be used without a tuner, and cost only a little more than $100 (including the PVC pipe, but excluding the antenna analyzer & coax).  Don't forget to add some kind of RF choke at the feedpoint to keep RF from snaking back down the 50 Ohm coax.  MFJ makes a compact little ferrite core model, but there are all sorts of wound coax low-cost designs out there.

There are still some things I’d like to try with this antenna system, but it’s working so well right now that I’m going to leave it alone and enjoy using it for a while.  Now that it’s 15’ shorter after adjustment, I’d like to put up two more masts to flatten this out into a full horizontal dipole at  14’.  I wonder how that would that change the 80 meter SWR curve?  That one 20’ PVC mast could be cut down to two flanged-together 7’ segments which would be far more portable.  On the other hand, it would take three of these 14’ masts.  Maybe I’ll do it if I get some free time this fall when the weather’s nice.  Another thing I’d  like to try is re-tuning everything to get the full 40 meter band.  I have some friends threatening to learn Morse and it would be nice to jump  in there with them.  But, like I said, it’s working well for now and I’m going to kick back and get some use out of it until the weather cools.

OK, project successful, and it’s a good antenna for regional communications.  On to other matters.

Here’s a link to the manufacturer’s site: http://www.mfjenterprises.com/Product.php?productid=MFJ-17758  Interestingly, they also make a 40/20 meter version too, which is only about 42 feet long.  While it wouldn’t do much on 20 meters if set up for NVIS, it would make a nice compact (if narrow-banded) 40 meter NVIS antenna.

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Offline TexasGirl

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #1 on: July 28, 2013, 12:27:53 AM »
Alan,

Which direction did you orient your antenna? (ie: east-west, NE-SW, etc.)  and have you noticed much directivity on or off axis?

~TG
 

Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #2 on: July 28, 2013, 06:26:43 AM »
TG, the wires are in a line almost exactly E-W.

No, I haven't seen any significant directionality for HF.  There may be a slight preference for N-S, but if there is, it's barely noticeable.  Farthest contacts I've made have been to central IL (650 miles), central FL (500 miles), and Houston area (not quite 400 miles), but since I haven't even filled one log book page that's not a very good statistical sampling.  Farthest I've received from was Santo Domingo, which is 1400 miles to the SE.

Now the odd thing is that listening on MW broadcast AM with this antenna, WOAI San Antonio comes booming in from the west, while it's almost deaf to WSM Nashville to the north.  OTOH, on shortwave WWCR 3.215 Mhz, also in Nashville, the signal is clear.  WSB Atlanta comes in OK, not great, and it's almost due NE.  This whole HF-omnidirectional / MW-directional-off-the-ends thing is interesting, but not a big deal since I'm not really using the antenna for MW listening.

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Offline TexasGirl

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #3 on: July 28, 2013, 11:19:21 AM »
Well, to be fair, WWCR 3.215 is 100,000 watts.  I can pick it up in Texas on a coat hanger. 

~TG
 

Offline armymars

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #4 on: July 28, 2013, 12:17:37 PM »
   As for adjusting the antenna. My favorite trick is to measure it after I put it up the first time , then readjust the equation I used the first time.  We all know that the standard formula is 468/fMHz = length in feet. So the first time I used the formula  I divided 468/7.275 and got 64.33 feet. When I checked the antenna I found it was on 7.150 because it was so low to the ground. So now I multiply 64.33 time 7.150 and get 460. Next time I try 460/7.275=63.22 feet. This saves a lot of time and work's great in speeding things up.  It now takes me only 3 tries to get most antennas where I want them.
   A flat top should raise the frequency a bit. Less coupling to the ground. At 20 feet an inverted Vee or flat top should have a near circular pattern and look like a bowling ball in the vertical plan.  73

Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #5 on: July 28, 2013, 02:30:38 PM »
Well, to be fair, WWCR 3.215 is 100,000 watts.  I can pick it up in Texas on a coat hanger. 

I think what may be happening is the raw voice power of Alex Jones drowning out WSM's Eddie Stubbs!

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Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #6 on: July 28, 2013, 02:37:51 PM »
A flat top should raise the frequency a bit. Less coupling to the ground. At 20 feet an inverted Vee or flat top should have a near circular pattern and look like a bowling ball in the vertical plan.  73

Thanks, that is good to know.  All else being equal then, I'd have to lengthen the antenna if I took it from an inverted Vee to a flattop, and I could easily run out of backyard at some point.

On a related note, this morning I found a couple of pdf's with some useful NVIS information, including field plots showing the circular pattern you describe:
https://www.txarmymars.org/downloads/NVIS-Antenna-Theory-and-Design.pdf
https://www.txarmymars.org/downloads/NVIS-Theory-and-Practice.pdf

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Offline armymars

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #7 on: July 29, 2013, 03:25:31 PM »
    Both web sites are one of the best sources for NVIS antennas. You have information there that took me 15 plus years to learn on my own. The first time I read about NVIS , it wasn't even called that. The antenna  was described in 73 Magazine.  It was built so you wouldn't have to listen to the broadcast stations in Europe and could use 40 at night. The ham built a 40 meter folded dipole 6 feet off the ground with 3 reflectors on the ground 6 feet apart.
    This was back in 68-70 or so. 
    When you make your dipole a flat top, it will afflict 40 more then 80. I think.  73

Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #8 on: July 29, 2013, 09:10:42 PM »
The antenna  was described in 73 Magazine.  It was built so you wouldn't have to listen to the broadcast stations in Europe and could use 40 at night. The ham built a 40 meter folded dipole 6 feet off the ground with 3 reflectors on the ground 6 feet apart.
I saw that!  It's a great article.  And the distant station filtering does seem to be working.

Here's a link to a downloadable copy: http://archive.org/details/73-magazine-1969-10 

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Offline armymars

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #9 on: July 30, 2013, 03:44:13 PM »
     Would you believe I have a my copy of that in my basement,         some place.  73

Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #10 on: August 12, 2013, 06:36:23 PM »
I had some ambitions to rig this antenna up for NVIS (inspired by this link http://www.hamuniverse.com/supernvis.html), but didn’t quite have room in the backyard for the full stretched-out length plus the end-poles and their guy lines.  So instead I rigged it as an inverted V with the center at 20 feet, because that’s the standard length of 2” PVC pipe and that’s what I had on hand, and ran three 22-gauge wires underneath, borrowing the reflector concept from that NVIS design page.

More research dug up this presentation:
http://www.w5jck.com/nvis/W5JCK-NVIS-Antenna-Presentation.pdf
Down around slide 30 (plus a few past there), there're some EZNEC model results showing that reflector wires don't help appreciably, but that they do jack up the SWR and narrow an antenna's bandwidth.  So I removed the reflector wires this past weekend.  No appreciable drop in performance, and nearly all of the 80 meter phone band is now useable.  And as a bonus, the grass is now that much easier to mow.

BTW, with more contacts now made, a significant north-south preference on the antenna's pattern is becoming evident.  But coverage is good enough east-west that I'm happy with it as is.

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Offline armymars

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #11 on: August 13, 2013, 08:18:06 AM »
If you were to raise the antenna 6 more feet then elevate your radials 6 feet off the ground you might see some different's. Elevated radials were tried on a vertical AM broadcast station in Maine several years ago they found 6 radials at 20 feet for 900 KHz worked as well as 120 one foot off the ground. There was a nice write up in World Radio about it. I think you can still fined it on the net.  73 

Offline idelphic

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #12 on: August 13, 2013, 01:52:41 PM »
While these are metal - I say them the other week and Lowe's and wondered about using them as mast sections..

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Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #13 on: August 13, 2013, 03:24:04 PM »
While these are metal - I say them the other week and Lowe's and wondered about using them as mast sections..

Concrete Float Pole

It'll probably work just fine.  After all, while mine has a PVC pole, there's still the metal outside of the coax shield braiding running straight down from the feed point.  Worst that can happen is a crummy sorta RF ground loop between the pole and that shield; that might cause a little extra noise.

And that aluminum pole is going to be a lot less bendy than 20' of PVC.  Mine's already showing an appreciable bow in it.

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Offline idelphic

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #14 on: August 13, 2013, 06:00:48 PM »
It'll probably work just fine.  After all, while mine has a PVC pole, there's still the metal outside of the coax shield braiding running straight down from the feed point.  Worst that can happen is a crummy sorta RF ground loop between the pole and that shield; that might cause a little extra noise.

And that aluminum pole is going to be a lot less bendy than 20' of PVC.  Mine's already showing an appreciable bow in it.
Well - I wondered about a hybrid - use them as the main mast, but then isolate the antenna from the mast using a Poly type or PVC section.
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Offline armymars

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #15 on: August 14, 2013, 07:09:41 PM »
   I have some of the military poles for camouflage nets. All but one are aluminium. I put the fiberglass pole on top. It seems to work well for me.  73

Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #16 on: October 04, 2014, 07:29:43 PM »
Re-tuned things today, and now the antenna has a nicely centered SWR curve on 40m, 1.5 at the 7.0 and 7.3 MHz band edges, with a smooth dip through the middle.  Its performance on 80m improved a little too, with SWR < 3 from 3.73 through 4.0 MHz.

Weirdly, it also will now tune on the 10m and 17m bands as well, using nothing more than the little internal tuner on my FT-450D.  I see some experimentation in the near future.

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Offline backwoods_engineer

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #17 on: October 08, 2014, 07:20:59 AM »
Good review, Alan.  I may get one of these for NVIS on 80.

Offline Carl

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #18 on: October 08, 2014, 10:44:44 AM »
Things I know are TRUE

An inverted "V" is OMNI DIRECTIONAL.as in without favor of direction.
Listening tests  of an antenna only tell you propagation trends
An antenna is depicted by strait lines,because they are easy to draw and print.
An antenna does not have to be straight to tune and work well
Stop complaining about life and start Celebrating it .

I've reached the age where there is little left to learn the hard way.

If you had only one year,one month,or one day...Would you live your life differently?

Radios are pointless without someone trained to use them.

Offline Alan Georges

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Re: MFJ-17758 80/40 Meter Dual-Band Dipole
« Reply #19 on: October 08, 2014, 04:31:24 PM »
Good review, Alan.  I may get one of these for NVIS on 80.
Thanks.  Here's the crazy thing: it'll tune anything above 30m now!  I don't know how efficient it is on some of these bands, but I did have good clear QSOs to CA and AZ on 15 & 17m last weekend, so it's at least working somewhat on these bands.

Now, tuning up on 15m isn't any mystery (15m's dipole leg length is an odd integer factor of that for 40m), but tuning 20, 17, 12, 10, and 6m sure were.  Ah, the mysteries of radio.  These are just nice bonuses, so I'll take them and use them and not worry too much about it.

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