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

OhSixTJ

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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.
 

jonwienke

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The correct formula is "the shortest practical length required to connect the radio and antenna". Unless you're doing a phased antenna array, the notion of a magic or "correct" coax length is a myth.
 

SteveC0625

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Jon is correct.

Here are some guidelines for mobile installs:

1. If you trim the coax, leave about a foot of slack at each end.
2. Never coil up excess coax. It creates, well, a coil, which will affect the transmit properties of your install and in a good way. Spread it out or trim it back.
3. If you encounter an airbag, route the coax between the airbag and the body of the vehicle. Make sure it will not inhibit the proper deployment of the airbag.
4. Route over sharp edges at your own peril. Smooth ‘em, pad ‘em, or avoid ‘em.
 
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OhSixTJ

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Thanks guys. I was under the impression that since the coax was the ground plane for a 1/2w antenna that it had to be a certain length. I appreciate the info!
 

nd5y

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2. Never coil up excess coax. It creates, well, a coil, which will affect the transmit properties of your install and in a good way.
This is and old CB myth that is BS.
Coiling excess coax can't affect the transmitted signal unless the coax is radiating. If the coax is acting as part of the antenna and radiating then you have other problems.
 

OhSixTJ

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I had about 2’ of coax coiled and it was affecting both TX and RX in my particular application. Uncoiled it does not cause problems anymore.
 

jonwienke

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Coiling coax forms a common-mode choke. If your antenna is working properly it will have no negative impact on performance. If coiling/uncoiling the coax changes performance, you have a bad solder joint, loose connection, or broken conductor inside the coax.
 

W5lz

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OhSixTJ
I have to think that you have other problems if uncoiling a little coax makes a significant difference in RX or TX signals. Then again, it depends on your 'particular application', huh?
 

jonwienke

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Thanks guys. I was under the impression that since the coax was the ground plane for a 1/2w antenna that it had to be a certain length. I appreciate the info!
A 1/2-wave dipole does not have a ground plane, so that's impossible. A 1/4-wave antenna has a ground plane, but the coax length doesn't matter if the antenna is set up correctly.

The origin of the myth comes from the fact that having a coax length an exact odd multiple of 1/4-wavelength (accounting for the velocity factor of the coax) fools an SWR meter into showing an inaccurately low SWR reading. But it doesn't actually improve antenna performance.
 

StaticDischarge

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I can already see that "can-o-worms" beginning to open but whatever... Sometimes there just has to be an end to the suffering...

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 and therefore, coiling the coax should make no difference, otherwise if it is cut to something other than odd multiples, the coax tends to become part of the antenna (as in the coax becoming a radiator), which is why the antenna may become difficult to tune and the reason why coiling of the coax has the effect it has... and it just so happens that the higher the frequency you go the more noticeable this becomes (think shorter wavelengths).. Oh and you can forget what I just wrote if cheap coax is used, the skill at attaching your connectors and how "technical" you really want to get about your antenna system...

And if somehow you still disagree, then why is coax used as tuning stubs and how do stubs work? And then you might also want to read up on phasing multiple antennas (think large array multiple beam EME stations) and why coax lengths, velocity factor, type of center dielectric and impedance are important factors to those systems... So with that said, if you choose to use this information you may just find out for yourself that you could have just had a V8!

Oh and it won't matter what type of "tuned" antenna you choose to use, either 1/4λ, 1/2λ, 5/8λ or some type of collinear makes not one iota of difference...

(λ means wavelength and "EME" is Earth-Moon-Earth for those who aren't in the know)
 

OhSixTJ

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OhSixTJ
I have to think that you have other problems if uncoiling a little coax makes a significant difference in RX or TX signals. Then again, it depends on your 'particular application', huh?
well my “problem” was some RF noise signal reports of “not making it into the repeater”. Now they say I sound just fine. I do have another coax on the way though.
 

OhSixTJ

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I can already see that "can-o-worms" beginning to open but whatever... Sometimes there just has to be an end to the suffering...

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 and therefore, coiling the coax should make no difference, otherwise if it is cut to something other than odd multiples, the coax tends to become part of the antenna (as in the coax becoming a radiator), which is why the antenna may become difficult to tune and the reason why coiling of the coax has the effect it has... and it just so happens that the higher the frequency you go the more noticeable this becomes (think shorter wavelengths).. Oh and you can forget what I just wrote if cheap coax is used, the skill at attaching your connectors and how "technical" you really want to get about your antenna system...

And if somehow you still disagree, then why is coax used as tuning stubs and how do stubs work? And then you might also want to read up on phasing multiple antennas (think large array multiple beam EME stations) and why coax lengths, velocity factor, type of center dielectric and impedance are important factors to those systems... So with that said, if you choose to use this information you may just find out for yourself that you could have just had a V8!

Oh and it won't matter what type of "tuned" antenna you choose to use, either 1/4λ, 1/2λ, 5/8λ or some type of collinear makes not one iota of difference...

(λ means wavelength and "EME" is Earth-Moon-Earth for those who aren't in the know)
not trying to open up a can of worms, just curious about coax length needed for a NGP antenna. 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.
 

jonwienke

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If anything acts as ground plane on a mobile antenna, it's the vehicle body. There is a 0% chance the coax is the ground plane.
 

StaticDischarge

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not trying to open up a can of worms, just curious about coax length needed for a NGP antenna. 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.
I'll be the first to admit this as I have zero experience with NGP antennas... So with that said and having lots of information stuffed away in my lonely brain cell I will guesstimate that a NGP antenna comes equipped with it's own ground plane, somehow.. Therefore it should be usable on a plastic/fiberglass body vehicle that doesn't provide a ground plane as a metal bodied vehicle provides..

So with that I will stand by what I stated previously which is you don't want your coax to become a radiator and that the type of antenna really doesn't matter as long as it is "tuned", so do the math and cut it accordingly... or have that V8...
 

jonwienke

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Coax radiating happens when you feed a balanced antenna without a balun, or improperly grounded antenna with an unbalanced feedline. When that is the case, the feedline will radiate regardless of its length. The correct solution is to install a balun or ground the antenna properly, not cut the coax to some "magic" length.
 

StaticDischarge

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Coax radiating happens when you feed a balanced antenna without a balun, or improperly grounded antenna with an unbalanced feedline. When that is the case, the feedline will radiate regardless of its length. The correct solution is to install a balun or ground the antenna properly, not cut the coax to some "magic" length.
Jon, Jon, Jon... Me's think you should just start liking the taste of a V8... We are talking about a mobile mounted antenna.. What, may I ask, is balanced about this style of antenna? And who do you know is so "smart" that they actually install a balun at the feed point of a mobile antenna for use at VHF or above??? Wait.. don't answer that last question.. I think I already know...

Again.. I am up against someone "way smarter than I"... Oh my...
 

jonwienke

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Most mobile antennas are 1/4-wave ground plane designs, and use the body of the vehicle as the ground plane. If you have a bad RF ground, the coax will radiate, regardless of how long it is.

"No ground plane" designs are typically modified dipole designs fed through the center of the lower half of the dipole, essentially making it a ground plane. So they can be fed with coax without the coax radiating, but again the coax length has no effect on whether the coax radiates, that depends on the antenna design and whether there are any bad/loose connections.

In neither case does coax length have any bearing on whether the coax radiates. That is determined by the antenna design and the quality of your connections.
 

StaticDischarge

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Ok then, whatever you say.. You seem to know exactly how RF radiates and why.. Makes me wonder why you choose to waste this superior knowledge on us here in this forum rather than earning the Nobel prize being the RF engineer you make yourself out to be??

Only if you read more than just between the lines that may help...
 

jonwienke

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I'm astute enough not to buy into old wives' tales and myths debunked decades ago. No Nobel prizes required...
 
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