Reception while driving

[HrXkb3UjmgFvBe]

I have a question about reception with a mobile radio when driving.

Is it normal for a signal (when listening to traffic) to fluctuate when driving and not when stopped or parked?

I have a mag mounted antenna and a Radioddity DB25-G GMRS connected to a cig lighter. The recieve signal can go from 60-80 and all in between, is this expected when moving in a car, specific in a medium sized city with may trees and buildings.

[Radioguy7268]

Even though we like to think of RF signal as "line of sight" and always traveling in a straight line, the reality is that the signal will bounce, refract, and even bend as it passes through and between free space (air) and objects on the way to your receiver.  

To understand why you're seeing differences in signal strength while you are moving (even slightly) - you need to understand multi-path and nulls. Basically, you are hitting spots where the signal is partially cancelling itself because two (or more) wavefronts are reaching the same point (your receiver's antenna) and they are out of phase with each other. When they're exactly 180 degrees out of phase, they'll cancel each other completely. 

The opposite is also true. When two wavefronts reach the receiver at the same time in phase, the net effect can actually be an increase in signal strength. That's part of the reason you'll find "sweet spots" where the radio works best in that exact location, even though you're miles from the transmitter.

If you ever suffer from insomnia, feel free to Google up Multipath and Diversity antennas and phase shifted noise cancellation. There's some interesting stuff that goes on in those areas, especially when it comes to data and higher (mm) frequencies.

 

 

 

[BoxCar]

Yes. Any changes to the terrain around you will affect recepton. You could have reflections from buildings or signal absorbtion when in the city. Trees will absorb signals in the country.

[mbrun]

Yes, and I agree with BoxCar. The further you are from the transmitter (repeater in this case) and thus the weaker its signal the more this phenomena will be perceived. The reverse is true also. Not only will you experience this when receiving, but others may experience this when you are transmitting. From time to time may hear on the radio someone tell another to stand still, stop moving or the like when their signal is varying to much to copy it well.


Michael
WRHS965
KE8PLM

[n4gix]

The phenomenon is known as "picket fencing..."

[MichaelLAX]

2 hours ago, Radioguy7268 said:

When they're exactly 180 degrees out of phase, they'll cancel each other completely. 

I remember when I was about 12 years old and my monthly copy of ARRL's QST magazine arrived.

It had a construction project that took advantage of that phenomena for HF; using out of phase signals to completely open up a frequency where a "pile up" was occurring due to a rare foreign country's Ham signal was being received.

It then had circuitry to allow "my" signal to make it through free and clear!

And then I looked at the cover of that issue and realized it was APRIL! 

[HrXkb3UjmgFvBe]

Thank you for all of the responses, I did not want to return or exchange the radio; i've done enough of that the last two weeks when I noticed I got either the wrong cable length or a couple of antennas not tuned for GMRS. I ended up buying most of my cables and antennas at buytwoway and returned all of my amazon GMRS related items since they where about the same cost (in most cases cheaper outside of Amazon and ebay) and I know that I would get the correct product.

[WyoJoe]

12 hours ago, MichaelLAX said:

I remember when I was about 12 years old and my monthly copy of ARRL's QST magazine arrived.

It had a construction project that took advantage of that phenomena for HF; using out of phase signals to completely open up a frequency where a "pile up" was occurring due to a rare foreign country's Ham signal was being received.

It then had circuitry to allow "my" signal to make it through free and clear!

And then I looked at the cover of that issue and realized it was APRIL! 

That's pretty funny!

[HrXkb3UjmgFvBe]

I returned the DB25-G GMRS because it appeared and felt cheaper than the 20G. Also it did not have the ability to save tones for different repeaters on the same freq. I am happy with my Wouxun KG-XS20G, reception is improved by a lot and I can program tones to repeaters in whatever order in the memory regardless of the GMRS freq...this feature is a must for me when I travel and have repeaters pre-programmed.

[PACNWComms]

Thank you for the warning on the DB25-G, I also like to have channels pre-programmed for my area. For many years this meant a Vertex VX-3200, as I had 128 channels and also used many of these for work on board a fleet of vessels for oil spill and emergency response efforts. With a smaller car now, I did buy a Radioddity DB20-G (the Wouxun would not have fit), as it actually fit a cubby hole in the dash perfectly. But, ruggedness is not the same as the Vertex. However, it can be made to work in most of the VHF/UHF band, which is useful to me now.