View Full Version : 2 meter dipole?
01-30-2009, 05:25 AM
had an idea while laying in bed for my wanting to talk on the satellites so badly syndrome, and thought about a 2 meter dipole set up horizontaly, which would lobe out, as well as up too.
anyone else ever try this for working the birds? how'd it work out?
i guesstimate using, length= 468/144.5 making it 3.24' overall, with legs being 1.62'
now the real question is, would i get any benefit out of making it longer in equal 1/2 wave lengths or would that not work.
i figure to make one and hang it up on my deck railing for a couple trial runs, and if it seems like it'll work, mount one up on the roof. and be able run it with the ic7000 qrp, which would be alot nicer than the ht. specially for the freq shifting.
also, could i make one that is dual band? for 2m 70cm? i assume a tuner would probally need to be used for that 1.
01-30-2009, 05:29 AM
read this on an article for a 2m/70cm dipole off arrl,, found herehttp://www.arrl.org/tis/info/pdf/0302038.pdf
a 150 MHz antenna can be used at 450 MHz. How-ever, the performance of the antenna at the third harmonic is poor when it is used in a vertical configuration.
would this be a factor beings i plan on being horizontal?
01-30-2009, 03:10 PM
Yes you can build a dipole for VHF and UHF and successfully work the satellites or through ISS. However there are better antennas for that purpose that are not all that much more complex.
If you are looking for simplicity and want an omnidirectional antenna that does not require pointing. The dipole does not exactly fit this requirement.
It will have nulls off the ends of the wires or elements. A loop antenna can be used and will have a more omni pattern, but usually the gain is a bit down.
Some antenna designs like eggbeater or texas potato masher have been developed that have omni pattern and have a bit more gain at the horizon where the losses to the satellite are greater. They dont have the null overhead like a vertical, but the pattern is kind of flattened overhead.
You can see a pretty good design of an eggbeater style antenna here.
As far as calculating the dimensions for a wire dipole at VHF, the rule of thumb formula will get you close, but it really is not very accurate. If you are planning on using the 12Ga copper wire you mentioned in your previous experiment then if you want it at 145 MHz each leg should be 19.8 Inches long. If you want it centered at 146 MHz then 19.7 inches each side. But it is not super sensitive and the SWR will be acceptable for either of these for operating over the whole band. If you are planning on using a different element diameter, it would be best to calculate the length based on the actual diameter used.
Using the same antenna for 2meter and 70cm is not recommended.
Using longer dipole than half wave just makes the null problem more severe.
01-30-2009, 06:01 PM
thanx for the time to reply with that info.
i'll give the web some searching for a couple days and see what i can come up with.
if i can figure a sufficient system i can mount, my world of sat working will open up wide with the ability to use my 7000 over my ht
01-30-2009, 10:09 PM
You can take it a few steps further. Make 2 dipoles and mount them at 90 degrees to each other. Stick them 3/8 wavelength above some chicken wire or other groundplane, and feed them with a phasing line. Makes a great omni LEO Sat antenna. I believe it's called a turnstile antenna. The ARRL had handouts on how to build one way back in the W5LFL STS-9 days.
02-01-2009, 05:02 AM
ok, thank you,, i'll check into it
02-01-2009, 12:49 PM
While looking for other things I came across a design for a 2m antenna that was also used for leo sat's.
It was similar to a 1/4w ground plane but replaced the 1/4w vertical section with a 3/8w rod and it had 4 - 1/8 w radials.. 30deg angle.
his dimensions were 64cm vertical and 20cm radials...but he had trimmed for a better SWR.
The claim is he can work Sat's that are at low elevations.
02-05-2009, 03:57 PM
A horizontal, square loop, just like the radiator element of a quad works OK. Try 590mm/23.23ins square, fed at a corner, not in the centre of an element, made from 10mm/0.4in tube. If the polarisation isn't quite right, turn the whole thing 90 degrees horizontally, the antenna will have equal horizontal and vertical polarisation at right angles to each other.
The impedance will be between 120 and 150 ohms, OK for running 2 parallel 75 ohm coaxes in parallel as a match if you have some. There are a few simple coaxial baluns on the internet as well.
Various other horizontal loops work quite well, including circular and Moxon but take a little more work to construct.