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Been thinking about something re: portable temporary antennas, particularly 80 and 40 meters. Say you can't put up a high dipole, so your antenna becomse a cloud-burner NVIS antenna. Good for short range, obviously.
Then consider a vertical. A full-size vertical would be prohibitive in a lot of situations, so then the question becomes...
How short can a vertical be and still be at least as good as a low dipole for distant communication? Especially if it's got a top hat?
A tophat is preferable above a coil, certainly in the bottom of the antenna.
If you are trying to keep it low, a hi - Q coil and tophat at the top, and 6 to 7 meters for 80 meters.( 20 feet or so)
I made the same for 160 and got reasonable results, the main important thing to remember with verticals is the ground, with a vertical dipole or corner dipole, inverted V or al L fed at the top might give good results as well.
But a pure ( shortened ) vertical really needs lots of ground wires...
ON4UN Low band DXing gave me lots of good tips, it's here on the H.D. as antenna book.
I used army glassfiber poles 4 feet long camo green that can be connected to make a longer pole, mine went up to 9 meters just with a few tie down lines at 1/2 way up.strong enough to carry a tophat and small coil at the top.
Not advisable for force 8 gale and above ;)
Thanks, Cor...good ideas to play with there. The question is, tho', as you compromise the vertical wrt height and radials, at what point do the compromises make the vertical as bad as a low dipole for DX?
Lots of things to play with, how is the ground under the antenna? rocky sandy dry. clay moist etc.
Radials are jsut one part of the equation, that will make the antenna more efficient.
The far field will be just as important as well, ( 10 - 100 times the wavelength) this will determine the angle you will get with the antenna length.
I live on young seaclay here, very good ground for H.F. the next better one is pure seawater.
That is how i get away with a mediocre L antenna and a not perfect radial field on the edge of the city and still can DX on 160 and 80 wityh the L.
Consider an L, horizontal/vertical polarisation, will give you NVIS contacts as well DX.
for 80, 10 meters vertical, 10 meters horizontal even if that is sloped down to 10 feet above the ground.
No need for an tophat, the horizontal section is the tophat and actually radiates, the tophat does not radiate...it is just the missing C for the too short antenna.
Currents in the wire sections of an tophat cancel each other.
If you use an tuner at the feedpoint you can use it from 80 and up as allband antenna, my L works from 160 - 10 meters.
I tried, and stepped away from using baluns/UNUN's at the bottom to feed them for multiband.
I know the 43 feet and 30 feet verticals are used that way, but i found the losses too big, a rasonable tuner fed at the bottom of the antenna has lots more efficiency.
Even an isolated wire thrown over a tree will work, an inverted U antenna ;) with 100 watts i never set the French countryside on fire with that :)
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=7479 forget the balun, and put tuner at the bottom.
Shock cord poles are pretty cool, and I have had similar thoughts, but prefer trees as a ready made solution.
MaxGain systems has cool fiberglass poles with thick walls that nest, maybe a bit long for portable use. http://www.mgs4u.com/fiberglass-push-up-mast.htm#38ft
http://www.mgs4u.com/pictures/RT-SPL-01.jpg
The "corner fed" inverted L dipole is a winner for camping.
I've also played with a sloping "vee" configuration for more gain in a certain direction. Tried it on a hill in front of a cliff once. Worked well, even without the terminating resistors to make a "half rhombic". I'd like to try it with the resistors sometime.
"Shock cord poles are pretty cool, and I have had similar thoughts, but prefer trees as a ready made solution. "
Yep....trouble is, in the particular state park that I was in there's a rule about hanging anything from a tree. Don't know the reason for that -- probably had people breaking branches off or something. So the thinking started from that point. Otherwise, a wire shot over a high branch will do nicely.
The question is an apples to oranges comparison.Quote:
Originally Posted by W1GUH
Low dipoles aren't good for distant communications. They're good for maybe the first 100-400 miles, depending on exactly how low in height we're talking about.
Verticals are DX antennas. They work best once you get past about 800-1,000 (and further). They stink below a distance of about 400-500 miles.
Ideally, you'd have both. If you can only have one or the other, then you need to decide what's the goal. If, for example, I want to talk to Pittsburgh from DE, I'd choose the low dipole on 75/80. If I want to talk to Europe on 75/80, I pick the vertical.
Nearly every antenna system is a compromise. You have to decide what compromises you're willing to put up with.
Thanks for all the info. Sounds like it comes down to try what's possible in a particular situation and see how it works. I usualy don't get all hung up on the theory, at least in the details. Seem to usually work out that sometimes the theory will say "Don't to that" where "that" is all that's possible, yet, if tried, will yield usable results.
As said, it's all compromise, on my hollidays to France i mostly don't know the situation either, so a 10.80 meter ( 9 sticks) glass fiber pole comes with me, and some nylon rope.
One OCF ( FD-4 ) for 80 -10 with rope, so i can use the almost 11 meter pole or as vertical, or use it as support for the OCF.
Lots of wire for the ground, 10 - 20 meter rolloed up each separate with crocodile clip, massive copperbronze 1/2 inch 5 feet 3/4 inch massive wire to pound in the ground ( if possible) and i'm set.
Till you 1/2 the antenna length you lose a few dB, goung shorter the losses add up fast.
have fun experimenting now ;)
Sometimes, a really low dipole will surprise you. I had a 45' tall 160m dipole that netted me 90 countries in one winter season. Being QRO helps lots, too.
Portable: Shoot a wire way up into a tree and hope it comes out both hort and vert.
Feed with a manual tuner.
Profit.
Four sections of mast, with a mobile antenna mount modified for dipole use. When I went on vacation at coastal Maine, I set this up using my car bike rack as a base. I was talking to my friend in Connecticut on 40M with an S20 signal.
http://paulfelgate.com/hamradio/dipole1.jpg
Note that the quick disconnect on lower left is isolated from the mast and mount. A second quick disconnect is mounted on the same plate, but pointing 180 degrees away. Coil of coax acts as RF choke. Note that the mobile mount is rotated 90 degrees. The hole pattern on the mount supports this.
http://paulfelgate.com/hamradio/dipole2.jpg
Two mobile whips of the same band is used. At $10 - $20 a whip, this is affordable.
My not-so-portable antenna
http://paulfelgate.com/hamradio/dipole0.jpg
Crosses three properties. The feedline center of the antenna is in one property near the corner where three properties meet. Strung between three tall oak trees, the antenna is high in the sky, and relatively free of tree branches, as seen by picture above. The blue line represent the ladder feed line.
I've used the "hamstick dipole" myself. Made mine out of PVC and simple stud mounts.
Works OK, but prefer the trees with wire when portable.
I've got a dipole mount and was all hot to try it untiil I realized something. A horizontal dipole, especially on the lower bands, is going to have a pretty high angle of radiation, while a vertical hamstick mounted on a car is going to have a lower (significantly?) angle of radiation. Given that, I had to wonder if I'd gain anything at all using the dipole mount. Seems that I'd be better off just leaving the antenna on the car if possible. If not possible, at least keep it vertical. This obviously depends on the particular application. Just may blow the $40 and have both set up as an A/B test.
Saw what may be an indication of this on FD. I was class 1C camped out in NW CT. A friend was with a group. I was using a hamstick on the car...he was using a low dipole. We were both on 80 CW.
Comparing notes the day after, he hadn't worked anyone outside of NE or NY, while I worked a lot of 4's...