Batboard

Does anyone know the difference in the dimesions between the 800Mhz and 900Mhz dipole duck antennas for the Jedi portables. I have some 800Mhz dipoles ducks and was wondering if I could cut about and inch or so off the top so that they may be resonant on the 902Mhz band....dan n2aym

Edit: I did some research and found out some more info - see below:

800 Dipole (8"): Red (NAF5039)

900MHz Dipole (?"): Blue (NAF4070) Item# 855884R02

If anyone has a 900 dipole I would appreciate a measurement.
Thanks.....dan n2aym

I have one of each here. The 800 measures about 7 1/4" overall, and the 900 is about 6 7/8". But I question the wisdom of adjusting the length of one end of a dipole. It'd probably be better to use it as is.

Jeff

W6JK wrote:I have one of each here. The 800 measures about 7 1/4" overall, and the 900 is about 6 7/8". But I question the wisdom of adjusting the length of one end of a dipole. It'd probably be better to use it as is.

Jeff

OK Jeff - tnx for the measurements.

Think about it this way. The ground end of these duck dipole antennas is the radio case itself. Correct me if I am wrong about that. If you have an
MTX9000 and a MTX8000 the case in each is identical. The only difference is the duck antenna and the freq range of the radio. The 800 & 900 dipole ducks are most likely very close in all of the dimensions except for the top skinny part of the ant ( at least it looks that way ). Thats the part I want to even up. Does that sound logical??? Dan n2aym

I have no knowledge of the internal construction of these antennas, but I can say that the design you're describing is a center fed one wavelength dipole, which would probably be difficult to feed. There's a reason why your typical diploe is one half wavelength long, and this is it.

Jeff

W6JK wrote:I have no knowledge of the internal construction of these antennas, but I can say that the design you're describing is a center fed one wavelength dipole, which would probably be difficult to feed. There's a reason why your typical diploe is one half wavelength long, and this is it.

Jeff

Being the curious minded tinkerer I am I tried slicing one open. These things are totally indestructable. I tried slicing with a razor from both seams one on each side and still cannot peel back the tough rubbery plastic all the way. I did get a peek at the base by the sma connector and it appears there is an outer helical coil that is grounded and an inner helical coil (up through the center of the grounded outer coil) that gets connected from the sma center conductor. The outer coil ends at the end of the taper and the inner coild extends up and beyond the outer coils limit. I ohmed it out and confirmed that. I am not sure what the electrical characteristics are of this physical configuration. Anyone know what type of dipole design this is?

Sounds like a simple coaxial dipole. The grounded side serves as the feedline also. The "helical coils" are actually tightly wound and uninsulated, to form a flexable cylindrical conductor. It'd be tough to shorten both sides of the dipole, and shortening one side would leave it unbalanced.

Jeff

OK - Thanks for the insight Jeff. I'll wait till I come across a couple of 900 dipole ducks. Its not real high on my priority list so it can wait for awhile....dan n2aym

On almost every elevated feed antenna, there is a short length of high quality, silver plated coaxial cable running up through the center, with the shield exposed and crimped to the exposed dipole ground 'radial' and the center coaxial cable is run up through that center to form the radiating element.

I've had these styles apart and there is very little difference in the ground radial design, only the length of the radiating element is different.

If you cut close to the base of the antenna, you should be able to separate the outer covering and expose the internal antenna elements, then tune/trim the radiating element to about 2" in length, a little more will not matter, but never less....it will give you a broader tuning range as well since the split is so wide, and 902 is closer to the upper 800's it shouldn't matter at all.

Even the 800 whips that look the same as the 900 variety work very well, and I have used them and tested them in a test setup that uses a separate ground plane and found the SWR to be almost exactly the same....low.

I can also use just about any 440 cut antenna on 900 due to harmonic relationship and get almost 'perfect' SWR readings.

Trial and error will prove things out as to what works and what doesn't.

tnx for the additional info ernest....

One additional 'addition' here, from trying out both styles...

The 800 labeled antenna does affect talkout range when used on 900, but I have not measured the whip diameter to see if this is the actual difference, but from outward appearances, in both length and width, they are identical...more tests required.

I also noted that there also appears to be a slight difference in overall length from the 800 whip to the 900 variety, and I noted about 3/16" to as much as 1/4" difference here, which can affect the radiation pattern whn using an 800 whip as a 900, but reception does not seem to suffer.

The 'ball' at the top of these whips actually covers a plastic 'washer' dropped around the braided whip, to retain the 'strands' that are wound together to form the actual whip itself, since this is of a spiral design, and soldering a metallic 'washer' to the whip would have an effect on the bandwidth of such an antenna, I can understand the reasoning behind the use of plastic for this application.