Im currently using an ST2 for an airband feed. Would I gain anything from an airborn standpoint switching to the Centerfire 1/4 or kb9vpr jpole?
Antenna question
- Thread starter dmoore12345
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Don’t know about air-band but will give thumbs up on the KB9VBR J-Pole.
Bought one tuned to 155mhz for public safety and I’m satisfied with it.
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A J-pole for 155 mHz would be pretty deaf on the aircraft channels. J type antennas aren't very widebanded. Even buying or building one tuned for the aircraft band would really only be good at most +/- 5 mHz of whatever the center frequency is (not enough for the entire civilian aircraft band), not to mention any J antenna will light up the coax with RFI. A tuned Centerfire 1/4 wave GP would do better although I have heard Centerfire is among the bottom of the barrel quality-wise, so YMMV. Your best bet would be a discone with 12-16 elements from one of the ham brands like Diamond, Comet, Jetstream, Tram/Browning.
Neither is ideal for either VHF or UHF aircraft.
A discone will be the best bet for several reasons. First, they are broadband and will cover both civil and military aircraft bands. Next, they have a higher angle of radiation which is good when the signals you are trying to receive are above the horizon. They have unity gain, but aircraft reception is almost always line of sight using relatively low power.
With the commercial discones you can remove the loading coil and spike whip if you are not interested in low band reception.
If money I not an issue consider Telewave discones. They aren't cheap, but are of extremely high quality.
J-poles have their uses, just not the best for aircraft reception.
With the commercial discones you can remove the loading coil and spike whip if you are not interested in low band reception.
If money I not an issue consider Telewave discones. They aren't cheap, but are of extremely high quality.
J-poles have their uses, just not the best for aircraft reception.
Ubbe
Member
If a discone has 8 or more bottom legs it doesn't differ performance wise between different discones made for the same frequency range. It's only the quality of components that are different, how they can handle long term exposure to weather. A standard discone works best at 100Mhz and at higher frequencies it tilts it beam upwards above the horizon, maybe a positve thing while aircraft monitoring and could be why you often see discones at smaller airfields. At the big airports I often see mostly single dipoles used for company frequencies to airplanes and similar communications that will have a beam reaching further at a horizon level where the maximum coverage are.
A j-pole antenna are a balanced antenna so when connected to an unbalanced coax you'll need to decouple it from the antenna, usually by a current balun. But why doesn't anyone use a 300/75 transformer balun and connect that to the j-pole at a point higher up where you have a 300 ohm impedance?
/Ubbe
A j-pole antenna are a balanced antenna so when connected to an unbalanced coax you'll need to decouple it from the antenna, usually by a current balun. But why doesn't anyone use a 300/75 transformer balun and connect that to the j-pole at a point higher up where you have a 300 ohm impedance?
/Ubbe
How much does the element diameter affect the bandwidth of say a VHF (150 MHz) J-Pole design?
I know with many antenna designs that a larger element diameter can increase the useable bandwidth of a given design but I don't think I've ever seen element diameter changes included in J-Pole design calculators.
The typical stated +/- 5 MHz of bandwidth for a J-Pole would probably work for most of my needs, a slightly higher bandwidth could be desired at times. J-Poles are pretty simple to build so if bandwidth can be increased +/- 2.5 MHz more by using larger diameter copper, that could be a plus in building one.
Anyone know the answer?
I know with many antenna designs that a larger element diameter can increase the useable bandwidth of a given design but I don't think I've ever seen element diameter changes included in J-Pole design calculators.
The typical stated +/- 5 MHz of bandwidth for a J-Pole would probably work for most of my needs, a slightly higher bandwidth could be desired at times. J-Poles are pretty simple to build so if bandwidth can be increased +/- 2.5 MHz more by using larger diameter copper, that could be a plus in building one.
Anyone know the answer?
Ubbe
Member
It's the same type of relationsship between diameter of elements and bandwidth as any other antenna type. It has to do with the wavelenght versus thicknes of element and thicker elements gives broader frequency range.
One problem is that it isn't as big a difference in a j-pole as with other antennas due to the design and the signal drops off twice as fast as other antennas when you go out from the tuned frequency. You could probably compensate for that by some sort of off center feed dipole technique with a reduced maximum gain but wider frequency range, by doing the tap of the coax braid match to a lower frequency and the coax center lead tap connected at a point that matches a higher frequency. But maybe the end result will equal a much smaller standard 1/4 GP antenna.
/Ubbe
One problem is that it isn't as big a difference in a j-pole as with other antennas due to the design and the signal drops off twice as fast as other antennas when you go out from the tuned frequency. You could probably compensate for that by some sort of off center feed dipole technique with a reduced maximum gain but wider frequency range, by doing the tap of the coax braid match to a lower frequency and the coax center lead tap connected at a point that matches a higher frequency. But maybe the end result will equal a much smaller standard 1/4 GP antenna.
/Ubbe
For my ICOM A25C, I have tried the Centrefire antenna (118-136 receive only version) as well as the DPD VHF blade antenna.....As far as performance.....the stock rubber ducky antenna that came with the radio is better.
A bit expensive but well worth the investment IMHO.
Diamond D-777 120/300MHz Airband Antenna sales Radioworld UK
Diamond D-777 120/300MHz Airband Antenna sales Radioworld UK
Ubbe
Member
USDipole makes a folded dipole for the aircraft band with a 300-75 ohm transformer. You might be able to get it without the transformer if you want to use 50 ohm coax and you offset it from your mast at the right distance.
VHF/UHF Folded Dipole Scanner Antennas
VHF/UHF Folded Dipole Scanner Antennas
majoco
Stirrer
I made my own from a cut-down TV antenna,,,,
I tried to make one but all it received was Jaz! 
N1XDS
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I am using a DPD Productions MilTenna Omni UHF/VHF Military Base antenna and it's one of the best antennas there is on the market. Dave the owner of DPD Productions is really nice and helpful on when needing help or advice on which antenna is right for you. I believe he is a forum member here on the forum as well.
MilTenna Omni Antenna
Ubbe
Member
Folded dipoles are always 300 ohm whatever you do with distances to mast, which only changes the beam direction.
/Ubbe
majoco
Stirrer
Most old TV antennas come with the 300/75 transformer - mine has it in the little orange box and it's fed with RG6.
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