Helical/Corkscrew in Mobile Antennas: What’s the high-level science explanation?

[CALO50]

Are they better than a straight mobile antenna of the same height (not length)? Is there a compromise? Is performance advantage material dependent?

[OffRoaderX]

I've always assumed that the corkscrew makes the RF electricities spin-around faster and then squirt out of the antenna at a higher rate, making them go more fars..

..i could be wrong though.

[CALO50]

4 minutes ago, OffRoaderX said:

I've always assumed that the corkscrew makes the RF electricities spin-around faster and then squirt out of the antenna at a higher rate, making them go more fars..

..i could be wrong though.

Ahhhh... RF electricities centripetal force.

[OffRoaderX]

My OTHER theory is that the corkscrew just allows you to have a shorter antenna with the same electrical length, and it has nothing to do with performance, other than having a shorter antenna that is just as performant as a longer one.

[CALO50]

2 minutes ago, OffRoaderX said:

My OTHER theory is that the corkscrew just allows you to have a shorter antenna with the same electrical length, and it has nothing to do with performance, other than having a shorter antenna that is just as performant as a longer one.

New theory...  RF electricities centripetal fars.

[marcspaz]

 

 

Okay, I will try to make this as painless as possible.  No promises though.  Hopefully, if I make a mistake, some of the smarter guys will let us know.

 

High-level Science

OffRoaderX is correct, but there is one other reason that is pretty common that comes to mind, specifically for phasing multi-band antennas.  Though, this obviously doesn't apply to GMRS specific antennas.

Quick example... the Diamond NR770 dual-band mobile antenna is listed as a 1/2 wave VHF antenna and a 2 phase 5/8 wave UHF antenna.  There is a coil in the center of the antenna to electrically make a VHF half wave antenna, which has almost no gain.  The coil also acts like a separator and a phase coupler(?) for each 5/8 wave segment, being the straight parts below and above the coil, leading to having about 3 dB of gain.

 

Is there a compromise?

eh... for GMRS, there is normally a benefit associated with it, such as having a gain antenna in a smaller package.  I am not really sure of any drawbacks if it built right.

 

Is performance advantage material dependent? 

Sort of.  Some materials conduct electricity better than others, but the common materials used for antennas have negligible performance differences.

[kidphc]

 
 
Okay, I will try to make this as painless as possible.  No promises though.  Hopefully, if I make a mistake, some of the smarter guys will let us ..

Lol...

Smarter???.. one of the few people that knows his shit. Then there is Mike W, when he talks, I just try to not look like a mouth breather next to him.

Sent from my SM-S901U using Tapatalk

[back4more70]

I see that my Nagoya UT-72G GMRS antenna does not have a coil, but my Tram 1181 VHF/UHF antenna does.  Hmm...

[marcspaz]

1 hour ago, back4more70 said:

I see that my Nagoya UT-72G GMRS antenna does not have a coil, but my Tram 1181 VHF/UHF antenna does.  Hmm...

 

On the Nagoya UT-72G, the coil in in the base of the antenna, referred to as a bottom load.  The Tram 1181 has a coil in the middle, referred to as a center load.

[CALO50]

19 hours ago, marcspaz said:

 

 

Okay, I will try to make this as painless as possible.  No promises though.  Hopefully, if I make a mistake, some of the smarter guys will let us know.

 

High-level Science

OffRoaderX is correct, but there is one other reason that is pretty common that comes to mind, specifically for phasing multi-band antennas.  Though, this obviously doesn't apply to GMRS specific antennas.

Quick example... the Diamond NR770 dual-band mobile antenna is listed as a 1/2 wave VHF antenna and a 2 phase 5/8 wave UHF antenna.  There is a coil in the center of the antenna to electrically make a VHF half wave antenna, which has almost no gain.  The coil also acts like a separator and a phase coupler(?) for each 5/8 wave segment, being the straight parts below and above the coil, leading to having about 3 dB of gain.

 

Is there a compromise?

eh... for GMRS, there is normally a benefit associated with it, such as having a gain antenna in a smaller package.  I am not really sure of any drawbacks if it built right.

 

Is performance advantage material dependent? 

Sort of.  Some materials conduct electricity better than others, but the common materials used for antennas have negligible performance differences.

Sounds reasonable. Thank you for not sending us into a differential equation coma. 

[WRUW493]

I'll give my take on it from an RF antenna engineer perspective...

In a few cases that I've seen the coil serves to make what should have been a longer antenna shorter. The classic example is a helical on a handheld, say for the 2m ham band, that should have been 1/4 wave, ie about 19 inches. The coil'ed up whip turned into a "rubber ducky" is electrically 1/4 wavelength long courtesy of the "mutual inductance" of the adjacent winds of the coil. Such an antenna is not 50 ohms but can be close-enough if the diameter/coil pitch and over all length are tweaked a bit. (did you know that a straight 1/4 wave whip is not really 50 ohms either, we shorten it a bit to bring it closer to 50 ohms). 

As for a coil in the center or there's abouts, of the mobile antenna: That can be to shorten it similar to the above. But most likely it's to give the proper phase relationship (read RF time delay) between the upper and lower straight parts of the whip. This is actually rather simple to think about if one comes to a curious understanding of how antennas work. Let me give that a short try....

The above diagram was pasted in from the web. The red line shows the magnitude (distance from the black line) and phase (which side of the black line) the RF current is on the various wire lengths (in fractions of 1 wavelength). Note that the top end is always zero magnitude. Has to be, at the end of the wire there is no where for the RF current to go, so one can literally start at the top of the whip at zero current and draw the above curves from the top down.

Let's look at the 1/2 wave example. If I could place two of these antennas, one on top of the other, then view this from a long ways away at the horizon to the side (where we want the radiation to go), we would see two RF current sources with the same phase and magnitude adding up in my desired direction. We call this "gain". ...gain being more signal in that direction compared to a dipole. 

But look at the 3/4 wave example above and note that the bottom 1/4 wave has the phase opposite of the top 1/2 wave (assume that the bottom extends below the horizontal line). Such an antenna does NOT have these two RF current sources adding towards the horizon because they are out of phase with each other. What to do???

The answer is simple: put a phase delay in between the bottom and top 1/2 wave sections so that they are in phase with each other. What could that be??? a coil would do this just fine. It would be good to make this coil physically rather small so that it does not radiate well itself, and of course would have to have enough inductance to perform a 180 degree phase shift in order to bring the top and bottom antenna halfs back to "in phase". 

This is just one example, but should give you the understanding that it is possible to implement this in a variety of ways with different lengths of top and bottom parts, each requiring different coils. Two 1/2 waves "co-linear" is classic and works well. But the coil has to be 180 degree phase shift and that's a lot of coil. More commonly they use say 5/8 wave over 1/4 wave which requires a smaller more practical coil, but does then suffer some undesired upwards radiation due to that annoying extra 1/8 wave part of the 5/8 wave section. Now we enter the world of compromises. Mobile antennas are full of compromises it turns out. The best antennas are the co-linear base station antennas that are housed within a protective casing (usually a fiberglass pipe) where they can implement the above idea of stacked 1/2 wave sections several times over and the fiberglass pipe supplies the mechanical strength. Some of those can reach 6dB and more in gain towards the horizon, something a mobile antenna can only dream of. 

I sure hope that makes sense and helps. 

[SteveShannon]

1 hour ago, WRUW493 said:

I sure hope that makes sense and helps. 

It really does. Thank you!