Nighttime Propagation on UHF??

[WRYC330]

I started GMRS only a few months ago. I have, however, been "DXing" both medium wave and shortwave since a kid so I am familiar with sunspot cycles, solar flares, the "K index", ionospheric bounce, F layers, etc etc. Does any of this influence GMRS/UHF frequency propagation?

I have two Chinese handhelds (specifically, a Baofeng GMRS UV5 and a Quansheng UV K6), and I am fortunately located at approx 1,350' elevation with a 40+ mile unobstructed view. Using either 5 watt HT with a rollup Slim Jim J Pole antenna tuned for 465mhz that I simply hang from the ceiling indoors, I can hit several repeaters 75 miles from my location during the daytime. After sunset or before sunrise, that distance increases dramatically. Last week, I was talking to a repeater after sunset atop Bagley Mountain in Lincoln Maine... which is 232 miles "as the crow flies" from my NH location. I initially thought this was an anomaly, but there is a consistent difference between my daytime and my nighttime ability to hit distant repeaters. I have discussed this topic over the air with these contacts, and everybody keeps telling me that UHF is "line of sight" and not affected much by solar activity, nighttime propagation etc. But my real world experience is telling me otherwise.

Anybody wish to shed more light on this topic??? I'd like to understand and better predict when conditions are ripe for GMRS DX.

   

[Borage257]

What you are experiencing is tropospheric ducting. It’s a wonderful thing, made a 92 mile confirmed DX with it earlier this year on simplex.

Here’s a forecast map:

https://www.dxinfocentre.com/tropo.html

And explanation

https://www.dxinfocentre.com/propagation/hti.htm

https://www.dxinfocentre.com/propagation/tr-modes.htm

[marcspaz]

As mentioned by @Borage257, especially due to the time of day and the time of year and what the forecast has been, you are near certainly experiencing tropo ducting.

 

That said, you touch on something that many people grossly overlook, including radio pros.  Although we discuss VHF and UHF radio being a "line of sight service"... that doesn't mean that atmospheric conditions impacted by solar cycle don't impact VHF and UHF... particularly in the case of GMRS.  In fact, during the day, they can have a profound affect in either increasing range or swinging the other way, causing radio blackouts.

 

UHF signals actually travel comfortably beyond the visual line of sight, by about 15%.  This is due to refraction and scatter affects in the atmosphere.  The more loose, electrically charged ion particles are in the air, the more scatter and refraction you get.  This translates to better signal quality at distance and often, a small bump in range.  The ionization in the F region increases roughly 2 × 10⁶ e/cm³ during maximum sunspot activity.  The F1 layer is the principle reflecting layer during the day in the summer.  This is why some bands perform much better during the day.

 

While these principles have more and more of an impact as you go lower in frequency, the lower the takeoff angle and the more the solar weather is spicing things up, the more likely you are to have a significant amount of your UHF signal refracted and/or scattered back to the earth beyond the horizon.

 

As an example, during a contest back in 2009, I was able to use a stacked beam array and 1,500w to make a contact on 146.52 MHz from a mountain top in Virginia to central Texas.  On that same day, I was also able to make a contact on 446 MHz to Tennessee, near the Arkansas state line, using the same kind of setup as the VHF contact.

[WRYC330]

Great stuff... thank you both for this excellent information. I was thinking of upgrading to either a 20 watt mobile (which I would supplement with an ac/dc power supply to use indoors), or even a newer 10 watt HT (specifically the Tidradio TD-H8 which I understand delivers a true 10 watts)... all to get some extra punch to reach distant repeaters. But from what you guys are telling me, its "tropo ducting" that is allowing me to reach out. A little additional power will probably not have much impact, correct?

[Borage257]

It might, I recommend mobiles because you can do cool stuff with them like hammo cans, base stations etc.

 

if you get a mobile, DB20G, RT95 are good choices for 25w radios

 

You’ll get more range from adding elevation to your antenna.