HF Fan Dipole Construction

[chgomonitor]

I'm currently using an end fed, random length longwire (~60 feet) for HF reception. I use Belden RG-58 for the lead-in, mostly for the shielding effect to reduce man made noise reception, (which I still get a ton of).

I've always been curious about Fan Dipoles, especially those constructed from 300 Ohm twinlead, (which is getting hard to find!).

I'm considering a 3-run fan using twinlead, to give me 6 dipoles. I plan on cutting mostly for the aeronautical & fixed services HF bands, which are my main targets, and I've mostly decided on the lengths. I figure I can probably cover 7.8 to 18 Mhz fairly well. (~60 feet overall is near my limit, though I'm almost 20 feet off the ground). I might try a 4 run / 8 dipole and toss in a broadcast band or two if I get brave.

At my local hobby shop I found some very tough, sturdy card stock in both polystyrene and even lexan which should work good for crafting a center tie point. I suspect I'll need to employ a 300 ohm to 75 ohm center balun to connect the feed line to, I plan to stick with the coax.

I doubt the Radio Shack 300 ohm twinlead will work well in this application as it's center insulation appears to be of the crumbly polystyrene sort, which wont work very well mechanically for mounting and cutting. I think I'm off to Menards and Home Depot to see if they still stock the old fashioned, flat, cheap, hard plastic stuff. I'm thinking of using some sort of outdoor, plastic adhesive tape to serve as element hangars / separators. Happily, I have a long driveway and rare decent Chicago weather today. I'd love to hang this thing up before the snow flies. I figure I'm going to need 140-200 feet of twinlead.

I know there's all sorts of dipole matching / element phasing issues with these things. But I'm willing to take a swing at that. My real concerns are more two-fold:

1. Has anyone been in a similar situation with replacing a random longwire with a fan dipole? How did it work out? Better reception? More or less noise?

2. Any mechanical contruction tips or hanging tips? At the center tie point I'm considering employing nylon bolts/screws/nuts and roughend washers to fasten down the twinlead - going through the middle of the insulation. Or, maybe a tie-bar arrangement.

I know the top element ends up supporting all the weight so I have to be careful with that one. I have decent ceramic insulators ready. I may have to reinforce my tie points / structures as this sucker is going to be A LOT heavier than my existing random wire.

If I make it and I don't killed I'll post results and/or pics later.

Any tips? Similar projects? Thanks in advance! - Ted

 

[gcgrotz]

Look at some commercial designs for ideas. Antenna Supermarket, Alpha Delta. Check Universal Radios web site. If you are making 1/2 wave dipoles (which it sounds like you are) then you will want a 1:1 balun, not 4:1 or 300:75 ohm TV types. You could get away with just a center insulator and no balun. I've seen these made of flat rotator control wire but I don't think that is available anymore. You can make spreaders out of 1/2" pvc water pipe with holes drilled in them. Use a dab of silicone (make sure it's not the corrosive kind) or hot-melt glue to keep them from sliding. And yes, be careful about the weight, that top element will have to hold it all up. You could use a good copperweld multi-strand wire for the top and smaller wires for the other elements. Check The Wireman web site, I've used the PVC insulated stranded wire for several projects and it is strong, tough stuff.

Good luck with it!

 

[chgomonitor]

Done

Thanks for the tips but the project is done (?).

My local Home Depots no longer carry 300 ohm twinlead. Neither did several other area electric supply contractors. Some were even unfamiliar with it! But I did find Magnavox brand twinlead in 75 foot rolls at Menards for 8 bucks a piece, bought three. They also had some generic, paper thin 50 foot rolls for 3 bucks which I shyed away from. As it turned out, the Magnovox stuff looks very sturdy - but isn't really. Very thin conductors and the heavy jacket actually splits and tears easily once knicked.

Due to space and tie down limitations I couldnt fan out the elements hardly at all. I found 3M brand super heavy duty outdoor duct tape which I used for element hangars/spacers and to reinforce the center tie-in point.

I got a sheet of quarter inch thick clear polystyrene (they were out of Lexan) and cut two 7 by 5 inch sheets. I used 1/8 x 1 inch bolts, washers and nuts to make a sort of tight sandwich center point. I drilled through the twinlead center about an inch from the element ends. The bolts go through both sheets of plastic and the holes in the twinlead, really gave a very sturdy center point between the mechanical connection with the twinlead and the compression from the plastic sheeting. I also used two bolts on the bottom as electrical contact points and soldered the centers of either side of the array to bus wire connected to these. I used a 300 ohm to 75 balun and am using RG6 as the feed. I used the wife's hot glue gun to seal the plastic sandwich and further anchor everything.

Construction turned out to be a bigger project than I thought, what with all the element measuring, trimming, cutting, drilling, gluing, taping, etc. Took all day yesterday. I hung it up early this morning with the weather rapidly worsening, always fun.

Putting the monster up proved to be quite a task. The only thing that worked to my advantage was the fact that everything proved to be a good deal lighter than I had anticipated. One side of it isn't quite hanging right, twisted around about halfway. I'm hoping gravity will eventually straighten it out.

I dont know if it is going to stay up there - the thing is a real wind catcher and I'm sure ice loading will also be a problem. We'll see! It's quite the wonderful eye-sore as well, LOL.

So how does it work compared to the old 60 feet random length end fed longwire? Apparently - fantastic! I'm not sure that the fan is as sensistive everywhere across the bands as the longwire was, but my noise floor dropped from S5-S7 to almost nothing. It's always hard to judge antenna performance casually on HF, but the fan appears to beat the longwire by a mile. I suspect moving the feed point out about 30 feet away from the house really helped the noise pickup situation. On and around the bands I cut it for it seems to be much more sensitive than the longwire.

Due to the lack of spreading out of the fan elements I'm sure I'm getting all sorts of phasing going on between the dipole elements - which are only spread out a couple of inches. But anyways, here's the bands I originally planned to cut it for. (I'm not sure I got everything right and I had to trim about one foot off one side of the longest element):

Top Run: 8 Mhz = 58.5 feet & 9 Mhz - 52 feet (total length - each side 1/2 of total)
Next down = 11.2 Mhz = 41 Ft 8 Inches & 13.3 Mhz = 35 Feet 2 inches
Next down = 15.5 Mhz = 30 Feet 2 inches & 18 Mhz = 26 Feet
Bottom run = 21.6 Mhz = 21 Feet 8 inches & 28.8 Mhz = 16 Feet 3 inches

These figures can be easily changed to the broadcast bands, I'm more into utility listening.

My total cost of construction was about 60 bucks, but I had the RG6 laying around. Overall, good project. Plus I cleaned out my gutters while up on the roof. Next I have to pickup all these fall leaves. Or, maybe I'll just listen to the shortwaves and let the leaves rot under the coming snow this winter. Good for the grass!

Happy Scanning! - Ted

 

[chgomonitor]

HF Fan Dipole & the Antenna Farm

Here's a pic (I hope...) - Ted

 

[chgomonitor]

Schematics of Center Point (pic)

Sorry for my crummy drawing skills...

 

[ka3jjz]

Ted, if this is an example of a crummy drawing skill, I'd love to see what you consider to be good. This is actually pretty clear, lots of good detail here.

This is the sort of stuff we can use on the HF Antennas Wiki. Nice, clear, detailed drawings. Perfect for someone else to come along and use, or perhaps improve upon.

73s Mike

 

[gcgrotz]

Good job Ted! You have related a very important lesson or two for all newcomers to HF monitoring. #1 is get the antenna outside away from the noise, and #2 is don't be afraid to experiment because you have very little to lose except maybe a few bucks and some time.

Way to go!

The only other thing I can offer is that if you are new to HF monitoring, the most important thing is to have patience, tune and listen. Signals don't just pop out at you like on a scanner.

 

[ampulman]

Sorry for my crummy drawing skills...

Not to worry about your drawing skills.

Have a question about the twinlead. At the feedpoint, you show each wire of the twinlead connected to the buss bar. What about the other end of the twinlead. Are the ends joined, and if so, does this offer any advantage over using a single lead? Is there a possibility of using each wire of a twinlead as a separate dipole, by cutting a notch out of one wire to shorten it?

Would converting each run of twinlead into a folded dipole be useful?

AM

 

[k9rzz]

Nice antenna. Seems like a lot of work though.

Why not just run the twin lead down from a 'normal dipole' (what ever length you have) and feed it through an antenna tuner? Then you can peak what ever freq. you want, it would work just as well, and be more apt to stay up under ice/snow loading.

Just my .02

John K9RZZ

 

[N4UYV_Al]

I run a 5-band (80,40,20,15 & 10) meter fan dipole with a balun in a 'squared S' configuration in my large attic (limited to no outdoor apartment antennas)..
The actual dipoles are 450 ohm ladder line cut at their perspective lengths fed by about a 25ft run of RG-8...This thing plays pretty good...

 

[kb2vxa]

Hi Ted and all,

That is in no way a fan dipole and why you used twinlead is beyond my imagining. OK, it works but here's the skinny on a real fan dipole.

Just to get it out of the way first, what you have is a parallel dipole and there is considerable interaction between the elements with one of these. You can't just cut them by the formula, that's only where you start. You have to prune them carefully using a wave meter each time to check for resonance until you get them right.

A fan is just that, a fan having the elements spread out. The uppermost really doesn't have to be horizontal, some can fan upward and give the overall structure greater strength. Ordinary 14ga stranded hard drawn copper antenna wire is not only cheap but far stronger and more durable than twinlead. You can use dog bone insulators throughout but center support insulators with SO-239 connectors are available and recommended since coax is heavy.

OK, leave it be for now because if it ain't broke don't fix it. When it comes down that's the time to put up a fan dipole and it probably will, twinlead won't stand the strain for long.

A note for Al, I hope you're not confusing window line with ladder line. Window line is twinlead with windows cut in it and in any case gets all upset when wet and pretty much craps out when laden with ice. Ladder line is open wire, that is it uses ceramic spacers and looks like a ladder. That stuff is quite unaffected by weather. BTW, I wouldn't want to go into your attic for fear of being attacked by the copper spider. I had an antenna like that in an attic and the spider ate the ventilator fan, ripped the windings out of the motor and started on the Romex before I subdued the beast. Antennas and copper spiders are best kept outdoors where they can munch on the neighbors' TV antennas and the occasional squirrel.

Note for John, tuners with switched inductors don't work very well out of the Amateur bands while those with roller inductors work well everywhere on HF. For SWLing something with continuous tuning is required and a preselector is preferable to a tuner.

Back to Ted, while coax may be used ladder line is the very best of any sort of transmission line you could possibly use in this application. There is one huge limitation, it must be kept away from nearby objects, metal in particular. It's more difficult to bring into the shack but it's been done for a hundred years, long before coax was invented. It helps to give it a gentle twist to minimize extranious pickup too and if you should decide to redesign that antenna system we can discuss things in greater detail.

 

[k9rzz]

Note for John, tuners with switched inductors don't work very well out of the Amateur bands while those with roller inductors work well everywhere on HF. For SWLing something with continuous tuning is required and a preselector is preferable to a tuner.

.

I've got a 20 ft vertical mounted on an 8 ft 2x4 with a dozen elevated radials fed with 'window line' and a roller inductor tuner .... it works very well across the board.

John k9RZZ

 

[chgomonitor]

Twinlead

Not to worry about your drawing skills.

Have a question about the twinlead. At the feedpoint, you show each wire of the twinlead connected to the buss bar. What about the other end of the twinlead. Are the ends joined, and if so, does this offer any advantage over using a single lead? Is there a possibility of using each wire of a twinlead as a separate dipole, by cutting a notch out of one wire to shorten it?

Would converting each run of twinlead into a folded dipole be useful?

AM

-The other ends are simply free (not connected)
-If you joined the ends you'd end up with folded dipoles all of the same length, so no bandspread
-Yes, there are other possible configurations. Folded dipoles actually work quite well, but are band-specific.

 

[chgomonitor]

Various replies

Yes, I know, it's a very minimal "fan" effect due to element interactions. As I said earlier, I'm limited in terms of both space and tie down points, so it works for me. All this talk of inductors, transmission lines, resonance, matching, etc is all well and good. But this is strictly for SWL work, I do no transmitting. I used twinlead because 4 runs gives me 8 dipoles and it's fairly light and cheap. Far too much of antenna knowledge (especially on HF) comes out of the amateur radio community. That's all well and good for hams, but the SWL is all too often left out in the cold. I guess my beef is that whenever anyone starts talking about a dipole or HF array there is an instant assumption that it is for ham use. Yes 450ohm ladder line is great stuff - if you are transmitting. As an unshielded feed in it picks of a lot of QRM, which is a real problem here locally. Anyways, I'm much concerned with receive bandspread, as in 20-30 Mhz. Yes, it's not sturdy. Yes, I'll probably rebuild it in the spring.

Does it work? Yes, great. It's local sunrise here and I'm listening to a MARS net on 3.793 (LSB). Stations across the country. Now according to "theory" I'm about 2 times away from any kind of match on this band. Basically, the "theory" doesn't matter a rats tooth to me, because I'M NOT TRANSMITTING and it works for me.

Nothing against hams - I "R" one too on VHF/UHF. Just tired of hams giving SWL's the techno-snub. And again, I do much more utility listening than broadcast stuff, so that's what I tried cutting for. Yes, I know it's not totally resonant in those bands either due to element interaction. Doesn't matter. Why? Because the issue is BANDSPREAD ON RECEIVE and not high gain in one spot and deaf somewhere else. I'm certain the element interaction is giving me ears in more bandspots than if the elements were widely fanned out.

Never been a huge fan of antenna tuners for HF receive use. They do work, I've used several. But my experience is a better antenna is always better than a poor antenna with a tuner.

Yes, far and away, and I'm dating myself here, the best SWL antenna I ever used was a vertical. At my old shack I had a Hustler Super Trumpet 5/8 wave CB vertical - gigantic. High 2nd story roof mount on a 6 foot tripod with a 6 foot mast and 3 full size radials. Fed with RG-8. (I was CB'er before the trucking craze hit). Used to work 5 watt skip all over the southern US on 5 watts from Chicago. Realistic Navajo Pro. Amazing. But the thing that really surprised me was how well it also worked on HF as a receive antenna from 5-30 Mhz. Highly sensitive with very little noise pickup or overloading - and that was at in the City of Chicago location up on the northside. QSL'ed 50 countries with an Allied SX-190. Worked great with my later Kenwood R1000 too.

This is another good example of a non-resonant antenna working great on receive outside of the band it's intended for. Verticals really cook on HF for receive.

In terms of sturdy, since I built it it's been through 3 wind storms, one up to 35 knots - Lake Michigan November gale. It's a real wind catcher - goes sailing around all over the yard, LOL. But so far it's intact. Okay, the end of one of the shortest elements is hanging out of it's duct tape a little. But so far it's surprised me - it's still up there!

When I do rebuild it I'll probably go with more of a proper fan shape design and perhaps use conduit or wire rope instead of the twinlead. On the other hand, if I'd done that I'd have ended up with 4 dipoles instead of 8, and less BANDSPREAD.

Start with "theory" for all your designs. Never be afraid to experiment or try something new. If you run into roadblocks, failures or mistakes (I made mine!) don't get discouraged. Try something new or find a work around. Be creative. And be safe working on antennas - always. Cheap ladders, thin roofs, tall places, electric lines, wasps nests are all hazardous.

But remember - if it works for you - that's what counts!

Happy Scanning! - Ted

 

[chgomonitor]

Oh and one other note. Gravity and wind did it's work and it's hanging correctly now.

 

[N4UYV_Al]

A note for Al, I hope you're not confusing window line with ladder line. Window line is twinlead with windows cut in it and in any case gets all upset when wet and pretty much craps out when laden with ice. Ladder line is open wire, that is it uses ceramic spacers and looks like a ladder.

Just goin' by what Van Gorden & AES advertises it as....

 

[kb2vxa]

Hi again,

From Al;
"Just goin' by what Van Gorden & AES advertises it as."

FLAME ON! That's just ONE of the things that galls me about the dumbing down of Amateur Radio, the gross misuse of terms. Manufacturers are false gods promulgating the incorrect usage of same, fooling people into thinking it's one thing when it's something else. Flame off.

Credit given to Stan Lee, Marvel Comics and The Torch.

Ted, if I were to use a pseudonym instead of my Amateur callsign as a screen name you'd never know the difference. I have been licensed but 11 out of the 40+ years I have been an SWL and I learned radio theory right at the outset in order to further my knowledge and enhance my enjoyment of the hobby. Amateur Radio and transmitting have absolutely nothing to do with one thing I have said so far. I'm dismayed by your attitude so common among the less technically inclined when theory has as much to do with receiving as transmitting. I try to teach you something and you throw it all away which is bad enough but doing so under false assumptions is much worse. This attitude makes me feel like I'm casting pearls before swine.

Sorry to come across like I'm coming down on you but that's simply how I feel about it. Perhaps some day you'll come to appreciate the value of knowledge which is essential for improving the hobby and your enjoyment of it. Until then I can only hope you don't become too frustrated with your hit and miss trial and error approach and I wish the best of luck to you.