I recently received this question via email from Dave N0MUA:

Bob, I ran across your pictures mountain topping on Colorado peaks, thought if anyone could answer this it would be you and your group.  We run on 146.52 here in Coffeyville KS.  and a group of us have brought up the question how far east can a mobile atop Pikes Peak be heard on 146.52 FM? The mobile would be mine running a Icom V8000 into a Tokyo HI power amp at 375 watts  LMR 400 coax to a Cushcraft 13B2 beam pointing to the east.

This is one of those how far will my signal go? questions that always gets my attention. Other folks may find this interesting, so I decided to spend some time on the topic and post it here. I am assuming we are talking about tropospheric propagation and not something more exotic such as meteor scatter or sporadic-e skip.

The (incorrect) conventional wisdom is that VHF propagation is “line of sight”, extending a bit beyond the optical horizon. From Wikipedia, we find that the distance to the optical horizon can be approximated by:

d = √(1.5 h)

where d is the distance to horizon in miles, h is the height of the observer above ground in feet

Pikes Peak reaches to 14,110 feet above sea level. The elevation of the surrounding area varies but since Dave is asking about propagation into Kansas, let’s use a typical elevation of eastern Colorado (4500 feet). This gives us an optical horizon equal to the square root of (1.5 x 9610) = 120 miles. Yes, this is an approximation, so feel free to knock yourself out with a more precise calculation.

It is interesting to note that there is a community 30 miles west of the Kansas border called Firstview, CO that is supposed to provide the first opportunity to see Pikes Peak when traveling from the east on Highway 40. Firstview is about 135 miles east of Pikes Peak, so the 120 mile calculation is in the right ballpark.

The ARRL web site says that the radio horizon is about 15% longer than the optical horizon, so that means our line-of-sight radio horizon is about 1.15 x 120 = 138 miles. I’ve operated from the summit of Pikes during the ARRL June VHF QSO Party and the Colorado 14er Event and working stations on 2 Meters at this distance is not difficult. To be more specific, I worked the Mt Sunflower crew (highest point in Kansas, 160 miles) from Pikes Peak on 2 Meter FM using a 25 Watt mobile and a not-very-well-positioned 1/4-wave antenna on the SUV fender.

I pulled up the distance records for the Colorado 14er Event and found that the best DX using 2 Meter FM is when Phil N0KE on Mount Bross worked Larry N0LL in Smith Center, KS at a distance of 375 Miles! Clearly, we are well beyond line of sight for this radio contact. N0LL has a very capable VHF station on his end and I believe N0KE was using a decent Yagi antenna and was running some power(~200W?). Still, this contact was on FM which is not that great for weak signal work. While Pikes is on the Front Range of the Rockies, Mount Bross (14, 172 feet) sits back some distance, about 60 miles west of Pikes Peak. (I have also worked N0LL from Pikes using 50W and a single 2M9 yagi on SSB with no problem during VHF contests.)

Also during the Colorado 14er Event, Phil N0KE (and Jeff N0XDW) on Mount Bross worked W7XU in Parker, SD on 2 Meter CW at a distance of 551 miles.  Keep in mind that as the signal strength fades, SSB has a serious advantage over FM and CW is even better! So for squeaking out the marginal contacts, CW is the way to go.

Dave asked about working Pikes Peak from Coffeyville, KS using 2 Meter FM. I had to look up where Coffeyville is and discovered that it is way the heck over on the east side of Kansas, maybe 50 miles from Missouri. I estimate that Coffeyville is 525 miles from Pikes Peak. To get back to Dave’s question, making a contact from Pikes to Coffeyville on 146.52 MHz FM is not likely. Maybe if we got some exceptional tropospheric propagation…but I think even then it would be unlikely to complete the contact using FM.

But you never know what might happen on VHF. That’s what makes it fun.

73, Bob K0NR