coax length for 1/2w mobile antenna on 70cm?

[Firekite]

Odd multiples of 1/2λ will help make the coax "invisible" to the antenna system and when this happens there is a better transfer of power which amounts to less loss

No. There is no world where a longer run of coax to his mobile antenna has less loss or “a better transfer of power” whatever nonsense that’s supposed to be. It does not matter the “quality of the coax“ or whatever else. Please stop spreading old ridiculous wives’ tales. It does not take an advanced degree in physics to understand that kind of magic is a myth easy to bust.

just curious about coax length needed for a NGP antenna.

As long as it needs to reach. That’s how long

Pretty sure I saw that the coax acts as the ground plane for these antennas but I could be wrong. Again, I’m talking mobile, whip antenna applications.

It does not.

 

[mmckenna]

I've been searching the web for a while now and can't find any info regarding the length of coax needed for a 1/2 wave NGP antenna. Anyone know of a formula or online calculator or if a specific length is even needed? This would be for 70cm frequencies. Thanks in advance.

This thread seems to be wandering all over the place. Since it seems like the original question has sort of been answered, but clouded with a lot of other info, I'll add my 2¢.

If you are mounting a 1/2 wave UHF antenna on top of a vehicle, with a proper NMO mount, then the roof of the vehicle becomes the ground plane. With a ground plane, the pattern of the antenna is different that if it was floating in free space or simply using the coax shield as part of the other half of the dipole. With the ground plane under the antenna, you'll see about 2.4dBd of gain. Without a ground plane, you'll see 0dBd of gain. The gain is achieved by the change in the radiation pattern of the antenna caused by the ground plane under the antenna. With out the ground plane, you'd pretty much have a dipole antenna with the whip being one half of the dipole and the coax shield being the other half of the dipole.

But I know, you were not asking about gain, but it does show that proper mounting location makes a difference.

As for "grounds", the 1/2 wave antenna does require a ground, but a ground and a ground plane are two different things. The outer shield of the coax will provide the ground back to the radio. If you don't have the antenna mounted over a ground plane, you'd still ideally want a 1/4 wave length of coaxial cable to act as the other half of the dipole antenna, although I've read things that suggest even 0.05 wavelength works. 1/4 seems to be the sweet spot. On UHF frequencies, you'd want about 6 inches. I wouldn't get too hung up on length, though. As was said, the length between the radio and antenna is the "ideal" length, no more, no less. But unless you are mounting the antenna right at the radio, that won't be an issue.

Any issues with the mismatch due to the lack of a ground plane will be worked out when you tune the antenna.

Really, a lot depends on where and how you are going to mount the antenna. I didn't see much in the way of specifics. I've mounted 1/2 wave antennas with no ground plane and less than a 1/2 wavelength of coax between the radio and antenna, and it made the SWR sweep a bit odd, but it worked well enough.

 

[OhSixTJ]

This thread seems to be wandering all over the place. Since it seems like the original question has sort of been answered, but clouded with a lot of other info, I'll add my 2¢.

If you are mounting a 1/2 wave UHF antenna on top of a vehicle, with a proper NMO mount, then the roof of the vehicle becomes the ground plane. With a ground plane, the pattern of the antenna is different that if it was floating in free space or simply using the coax shield as part of the other half of the dipole. With the ground plane under the antenna, you'll see about 2.4dBd of gain. Without a ground plane, you'll see 0dBd of gain. The gain is achieved by the change in the radiation pattern of the antenna caused by the ground plane under the antenna. With out the ground plane, you'd pretty much have a dipole antenna with the whip being one half of the dipole and the coax shield being the other half of the dipole.

But I know, you were not asking about gain, but it does show that proper mounting location makes a difference.

As for "grounds", the 1/2 wave antenna does require a ground, but a ground and a ground plane are two different things. The outer shield of the coax will provide the ground back to the radio. If you don't have the antenna mounted over a ground plane, you'd still ideally want a 1/4 wave length of coaxial cable to act as the other half of the dipole antenna, although I've read things that suggest even 0.05 wavelength works. 1/4 seems to be the sweet spot. On UHF frequencies, you'd want about 6 inches. I wouldn't get too hung up on length, though. As was said, the length between the radio and antenna is the "ideal" length, no more, no less. But unless you are mounting the antenna right at the radio, that won't be an issue.

Any issues with the mismatch due to the lack of a ground plane will be worked out when you tune the antenna.

Really, a lot depends on where and how you are going to mount the antenna. I didn't see much in the way of specifics. I've mounted 1/2 wave antennas with no ground plane and less than a 1/2 wavelength of coax between the radio and antenna, and it made the SWR sweep a bit odd, but it worked well enough.

thanks for your input. I did, unfortunately, forget to mention that this would be mounted on a UTV that does not have a metal roof. It would be mounded on a roll bar clamp that allows an NMO antenna to be fitted to it.

 

[prcguy]

I'll chime in to say most NGP antennas are 1/2 wave end fed with some maybe being 5/8 wave. There is a matching circuit in the base to transition from 50 ohms to high impedance and there is usually some common mode rejection built in to minimize feedline radiation. Some designs are not perfect and can be a little sensitive to coax length and in those cases a certain length of coax may show a better match than another length.

If I was using an antenna like this I would put a few snap on ferrite beads on the coax near the antenna mount or wrap maybe two turns of coax through one for the VHF/UHF range. A #43 mix or #61 mix ferrite should be good for this.

 

[mmckenna]

thanks for your input. I did, unfortunately, forget to mention that this would be mounted on a UTV that does not have a metal roof. It would be mounded on a roll bar clamp that allows an NMO antenna to be fitted to it.

You should be just fine.
I have a VHF half wave mounted the exact same way on a Polaris Ranger. Probably about 8 feet of coax down to the radio. Works just fine and tuned up nicely.

 

[OhSixTJ]

You should be just fine.
I have a VHF half wave mounted the exact same way on a Polaris Ranger. Probably about 8 feet of coax down to the radio. Works just fine and tuned up nicely.

Awesome. Can’t wait to get it installed.

 

[W5lz]

All thosem so-called NGP antennas do have a 'second half' which amounts to a "groundplane". That 'groundplane' may be the feed line (quite common). If it's mounted vertically that 'other half' of the dipole acts as a 'groundplane', it's what the 'top half' "works" against. The biggy between a vertical dipole and a "groundplane" antenna is that the bottom half is usually divided into multiple parts and fanned out to produce a differently shaped radiation pattern. Big difference in that patter from a vertical dipole? Not really, just good salesmanship, you know?

 

[WA8ZTZ]

There really aren't any perfect mobile whip antenna mounting options on a UTV or 06 TJ so do the best you can
with what you have and in the words of the old song... "whatever will be will be".
You really won't know until you try it out.
Let us know how it finally works out for you.

 

[prcguy]

Not really, see our fun conversation on this in another thread about HF EFHW antennas. There is a matching circuit in the base of the antenna that excites the half wave radiating element. There may be some or virtually no RF currents flowing on the coax depending on the design and if there are no RF currents on the feedline, then its not part of a ground plane or counterpoise.

In fact you can take a field strength measurement on most of these type antennas, choke the heck out of the feedline right at the antenna so there is absolutely no current flowing, and the antenna radiates the same, proving the coax is not or does not have to be part of a counterpoise. There is also stray capacitances between the matching circuit, radiating element and other things that help complete a circuit to satisfy the physics folks.

All thosem so-called NGP antennas do have a 'second half' which amounts to a "groundplane". That 'groundplane' may be the feed line (quite common). If it's mounted vertically that 'other half' of the dipole acts as a 'groundplane', it's what the 'top half' "works" against. The biggy between a vertical dipole and a "groundplane" antenna is that the bottom half is usually divided into multiple parts and fanned out to produce a differently shaped radiation pattern. Big difference in that patter from a vertical dipole? Not really, just good salesmanship, you know?

 

[W5lz]

In a 'perfect world' You would be absolutely right! Unfortunately... things seldom work out that way. The name, "groundplane" basically is a reference to a monopole being 'worked' against ground/dirt. So, if we made an 'artificial' ground, and moved the whole thing up higher, then it would work better, right? Get it above stuff that would block signals coming-n-going? Gee, lets try it and see what happens...