Rx/Tx loss with whip at driving speeds???

[amoking]

When I'm on the hwy, my Larsen whips (2m Rx/Tx and Low Band Rx) bend back about 45 to 60 degrees due to the wind. Does this flex impact the performance of the antenna?? The guy at HRO said no but my gut says otherwise. Compare to -for example- a Firestik CB antenna that one would see on a semi which doesn't bend.

 

[mmckenna]

When I'm on the hwy, my Larsen whips (2m Rx/Tx and Low Band Rx) bend back about 45 to 60 degrees due to the wind. Does this flex impact the performance of the antenna??

It would slightly. Similar to rotating an antenna from vertical to horizontal polarization. On long 5/8 wave VHF whips you can often hear the signal strength waver in and out a bit as the wind moves the antenna.

Really thin whips don't move around as much as a thicker one.

The guy at HRO said no but my gut says otherwise.

I'd trust your gut over HRO.

Compare to -for example- a Firestik CB antenna that one would see on a semi which doesn't bend.

Right,

Or the guys that cant them forward at a 45º angle to look cool?

 

[amoking]

So how, my friend, do we negate? Nature of the beast?

 

[prcguy]

In free space a 45deg slant to either side will loose exactly 3dB. Slanting towards or away is different and will depend on the radiation pattern and 3dB beam width and for a 1/4 whip its probably close to 3dB.

 

[mmckenna]

So how, my friend, do we negate? Nature of the beast?

Well, not sure you realistically can.
Use a thin whip to reduce windage.
Drive at a slower speed.
Switch to a 1/4 wave antenna for VHF
Use a stiffer whip
Use fishing line to make guy wires (dorky option)
Or, just live with it, like the CHP does.

 

[amoking]

Use a stiffer whip

I wondered, what if you got a small pvc sleeve and slid it over the mast. not sure what it would do to the signal.

 

[mmckenna]

I wondered, what if you got a small pvc sleeve and slid it over the mast. not sure what it would do to the signal.

Probably no noticeable impact.
I've put heat shrink over whips before.

 

[jonwienke]

Increasing the diameter of the whip will increase the wind load.

 

[prcguy]

I think you want a thicker whip to reduce bending. I have two Laird wide band CB antennas that come with a skinny long whip, something like 68" which bend a lot at highway speeds. I came across a similar length whip about twice as thick and drilled out the mounting hole in the coil and its a lot stiffer and straight at highway speeds.

Well, not sure you realistically can.
Use a thin whip to reduce windage.
Drive at a slower speed.
Switch to a 1/4 wave antenna for VHF
Use a stiffer whip
Use fishing line to make guy wires (dorky option)
Or, just live with it, like the CHP does.

 

[mmckenna]

I realize I left one out:

Drive fast enough that it folds over 90º and enjoy horizontal polarization.

 

[Ubbe]

I had taxi cabs driving mostly on the highway between city and the airport that where using 5/8 VHF antennas that complaint about their dispatch radio not working. Switching to 1/4 wave antennas with a stiffer spring fixed that problem.

Whenever you increase gain in an antenna you have to be much more careful of the angle of the antenna and in some scenarios gain antennas are not the best solution.

/Ubbe

 

[KB4MSZ]

According to Steven Wright you could drive at the speed of light and render the antenna useless in the forward direction.

 

[Ubbe]

According to Steven Wright you could drive at the speed of light and render the antenna useless in the forward direction.

As soon as you start to travel the doppler effect will shift the frequency and going too much off frequency will totally ruin reception, both forward and backwards in the direction of travel. It's actually an ongoing problem with modern super high speed trains and more problematic the higher up in frequency.

/Ubbe

 

[jonwienke]

I'm a bit skeptical of that last claim, given that aircraft typically go significantly faster than the fastest trains, and Doppler effects aren't an issue for aircraft comms. Doppler is an issue for satellite comms, such as using the ISS cross-band amateur repeater, but the ISS is moving about 100x faster than any train.

 

[MtnBiker2005]

Put a large shark fin cover over the whip. It will keep the whip from going backwards/forward/sideways
It should look good on top of the car.
Just be careful when it’s windy outside at highway speed.

 

[spacellamaman]

When I'm on the hwy, my Larsen whips (2m Rx/Tx and Low Band Rx) bend back about 45 to 60 degrees due to the wind. Does this flex impact the performance of the antenna?? The guy at HRO said no but my gut says otherwise. Compare to -for example- a Firestik CB antenna that one would see on a semi which doesn't bend.

my experience has shown the bending at speed is really not a problem.

on the other hand, if youre like me (oh you poor bastard) and have a 48" whip mag-mount on top of your honda civic, when you significantly slow down and/or come to a complete stop, reception suffers for close to a minute while that sucker whips around the top of the car.

you'll be busy trying to ignore the stares and jeers of other drivers tho, so you'll never notice any reception issues.

 

[spacellamaman]

Put a large shark fin cover over the whip. It will keep the whip from going backwards/forward/sideways
It should look good on top of the car.
Just be careful when it’s windy outside at highway speed.

hmmm not a bad idea. know where i could get a 48 inch one?

the great white whale they called him

 

[AI7PM]

As soon as you start to travel the doppler effect will shift the frequency and going too much off frequency will totally ruin reception, both forward and backwards in the direction of travel. It's actually an ongoing problem with modern super high speed trains and more problematic the higher up in frequency.

/Ubbe

Soo,.......how is it I was always able to communicate just fine with SR-71s above 60,000 feet going,...ehh....., faster than any train ever will, with no degradation of comms on UHF FM?

 

[Ubbe]

Soo,.......how is it I was always able to communicate just fine with SR-71s above 60,000 feet going,...ehh....., faster than any train ever will, with no degradation of comms on UHF FM?

Where you communicating with someone in an airplane approaching you? Analog FM will just change the modulation so the person sounds higher or lower in pitch. If that's happening to a digital modulation it's bye bye communication.

/Ubbe

 

[jonwienke]

Where you communicating with someone in an airplane approaching you? Analog FM will just change the modulation so the person sounds higher or lower in pitch. If that's happening to a digital modulation it's bye bye communication.

/Ubbe

LOL that's totally wrong. RF Doppler shift has no effect whatsoever on the pitch of demodulated FM audio, nor the pitch of demodulated AM. It will affect the pitch of demodulated SSB, but that's it. That's why you always have a clarifier control to fine-tune the RF frequency on a single-sideband radio, but it's not necessary on AM-only, FM, or digital radios (which typically use 4-state FSK, a variant of FM).

The only way you lose comms is if the Doppler shift moves the IF signal out of the filter window, and that typically requires orbital speeds. At 512MHz, a 6.25KHz Doppler shift requires a speed of 3,660m/s. That might be an issue talking to the ISS, but isn't going to present a problem talking to wheeled surface vehicles under any plausible circumstances.