Maximum GMRS Antenna Height

[Muzic2Me]

On 1/11/2021 at 8:08 PM, mbrun said:

I finally made the purchase of the HD 46’ push-up fiberglass mast I referenced in my earlier post. This weekend I rigged things up in a temporary way to conduct tests with local (and not so local) repeaters, as well as do some local simplex work.

 

The image below shows my antenna at full elevation (56’ AGL to tip of Antenna). Mast was installed in a temporary way just for testing. Eventually it will get moved next to the house. Amazing how plumb the mast was being guyed only at 7’3” (not good long term, but good enough for testing).

 

The antenna is a Comet 712EFC Collinear 9dBi.

https://www.dxengineering.com/parts/cma-ca-712ef

Cable is TMW LMR400

Mast is from DX Engineering.

https://www.dxengineering.com/parts/dxe-tfk46-hd

 

All of my testing consisted of using two handheld 5w radios. One feeding the Comet antenna, one in the car using an MXTA-26 antenna.

 

First, the high-point of the testing. I was able to communicate from my home southeast of Cincinnati to the Dayton 700 repeater located 50 miles north(straight line distance) using approx 3 watts of input power into the antenna. Signal report from a Dayton area resident was “very good” and “into the repeater solid”, perfectly suitable for a QSO. That alone made my weekend.

 

On the local front I was able to get into three local repeaters located 20, 21 and 26 miles way. Got in with full quieting to the one 20 miles away (“just as though I was next door”), and with just a hint of noise on the ones 21 and 26 miles away. Getting the antenna up high obviously makes a huge difference.

 

On the simplex front I found I could achieve pretty reliable coverage out 4 to 5 miles where things transitioned to hit and mis out to about 7-8 miles when all seemed to die completely. I expect the reliable range will decrease as foliage comes back. Not bad though for a couple of lower power HT and with good external antennas.

 

It will be nice to experience the positive change to simplex coverage after I up the power of the base and mobile to legal limits.

 

 

Michael

WRHS965

KE8PLM

Curious...So, you were able to hit a repeater 50 miles away on 3 watts. but not able to talk simplex 7-8 miles?  Just curious to why you could reach repeater that far away and not farther on simplex. Same wattage for the same distances?

[mbrun]

Curious...So, you were able to hit a repeater 50 miles away on 3 watts. but not able to talk simplex 7-8 miles?  Just curious to why you could reach repeater that far away and not farther on simplex. Same wattage for the same distances?

Quite simple…obstacles. Trees, homes, water towers, rolling hills.

The repeater antenna is somewhere around 900’ higher in elevation than mine. If you connected my antenna to the repeater antenna with a string, it would pass through very few trees and likely very few or no buildings. In contrast, when talking simplex (mobile or HT to base), the signal needs to pass through many thousands of trees, homes commercial buildings other obstacles.

You will hear over and over that GMRS is line so sight communications. This means if you have clear line of sight (i.e. your antennas can literally see one another) you can talk great distances. However, every obstacle you put between your transmit and receive antenna attenuates your signal and thus lessens your effective distance. The worst of these is the earth.

Midland gets away with misleading advertising of ‘37 miles’ because they actually can go 37 miles…if both radios are in outer space where there are no obstacles. Hear on earth…typically 1/2-2 miles.


Michael
WRHS965
KE8PLM

[JohnE]

I always referenced between two mountain tops in a vacuum and then it was a maybe. 

[Sshannon]

Even moisture in the air causes attenuation of UHF signals. 

[PACNWComms]

4 hours ago, Sshannon said:

Even moisture in the air causes attenuation of UHF signals. 

This is huge in my part of the Pacific Northwest....along with "pine" needles (pine being used for all conifer frees). I have had many people think that the range mentioned on many GMRS radios sold in pairs is realistic. UHF comms are always impacted in conifer forests and wet environments, yet it is also somewhat cheaper than 800 MHz when you have to buy spectrum and hope to get a license. With the telco's and public safety using that spectrum, many commercial enterprises, hobbyists, and others have to resort to UHF, or even VHF in this area once again, as it was pre-1980's. Then they find that going down to 450-470 MHz is not working in the forest or during rain, unless the systems can compensate automatically (P25 digital GTR based system), or going down further to VHF. 

Back to GMRS, the combination of marketing and environmental impact to signal propagation causes a lot of issues for those that try to us GMRS for their businesses, hiking in the wilderness (mostly conifer forests), and others in a similar frequency band. This is where power comes into play, and optimizing installation practices. It is common to find coax connectors corroded, or heliax filled with water if connections are not protected properly. Great point on attenuation at UHF frequencies, this is something many people do not even realize or prepare for.

[gman1971]

On 1/27/2022 at 10:30 PM, Muzic2Me said:

Curious...So, you were able to hit a repeater 50 miles away on 3 watts. but not able to talk simplex 7-8 miles?  Just curious to why you could reach repeater that far away and not farther on simplex. Same wattage for the same distances?

Radio waves coming from the antenna are just like the light coming from a flashlight. If you light it up atop of a lighthouse, someone will be able to see your light from tens of miles away, now, if you light up the same flashlight at ground level, it won't be seen from very far away at all.

Repeaters are like lighthouses, they are placed high above the ground.

G.

[gman1971]

On 1/29/2022 at 7:11 PM, PACNWComms said:

This is huge in my part of the Pacific Northwest....along with "pine" needles (pine being used for all conifer frees). I have had many people think that the range mentioned on many GMRS radios sold in pairs is realistic. UHF comms are always impacted in conifer forests and wet environments, yet it is also somewhat cheaper than 800 MHz when you have to buy spectrum and hope to get a license. With the telco's and public safety using that spectrum, many commercial enterprises, hobbyists, and others have to resort to UHF, or even VHF in this area once again, as it was pre-1980's. Then they find that going down to 450-470 MHz is not working in the forest or during rain, unless the systems can compensate automatically (P25 digital GTR based system), or going down further to VHF. 

Back to GMRS, the combination of marketing and environmental impact to signal propagation causes a lot of issues for those that try to us GMRS for their businesses, hiking in the wilderness (mostly conifer forests), and others in a similar frequency band. This is where power comes into play, and optimizing installation practices. It is common to find coax connectors corroded, or heliax filled with water if connections are not protected properly. Great point on attenuation at UHF frequencies, this is something many people do not even realize or prepare for.

Dang conifers.... but don't despair... never underestimate what some extra dBs can get you... 500kW on a highly directional antenna will probably melt the conifer forests, and anything in your path to reliable comms... so... problem solved... range will be assured.

VHF seems to outpeform UHF nearly on all scenarios I've tried. However, the VHF noise floor in certain areas around town gets high enough that its sometimes challenging to get reception, even for signals as strong as -90 dBm, and especially around traffic light controlled intersections, where you get some serious RFI on those dang traffic lights... UHF seems to be much less affected, but then again, you have other things that eat up UHF like its going out of style... like those conifers... or water... rain... etc.

G.

[PACNWComms]

11 hours ago, gman1971 said:

500kW on a highly directional antenna will probably melt the conifer forests

I used to be responsible for a fleet of AN/TRC-170 Troposcatter terminals, 6600 Watts would drop birds and kill trees (including pine trees) in the distance when used in Line of Sight (LOS) mode. At Timberline Lodge, I would rope off half the parking lot, and then see my signal drop when some fool would run over the flagging tape, knock over a dozen traffic cones and park their larger than school bus sized recreational vehicle right in front of my "Mickey Mouse Ears" antenna system. SatCom was a lot easier in that regard. After a week, there would be a path of dead birds and brown/red conifer tree in the distance....I did not like working LOS mode with those terminals. (Smarter birds would leave the area, it was always the robins and finches that were found dead, no eagles or crows.)

Living in a pine forest then resulted in the development of UHF satellite communications, because, as you mentioned VHF has become saturated in many areas. VHF is cheap, but abundant, and I see that as well with UHF. UHF is also saturated (partially due to the proliferation of CCR's), and so is 900 MHz ISM band frequency hopping radios, as they were sold to every office and construction company in the region. That is also why I added a DTR410 to my suite of radios that I use to monitor local comms. 

All of this is driving many users to higher end and more selective radios though, which I see as a good thing. For GMRS, antenna height and quality can help, but so can a better radio. My minimum is Motorola CDM1550LS+ mobiles and HT750/1250 handhelds (but would use CDM1250's for mobiles if I owned any). It is all about the system or package, which can result in the system becoming more than its parts, if a person does not cheap out or cut corners. While CCR's may get people into the hobby, better gear will help, as will height and quality coax and antennas.

Some of these forums have made me laugh with the justification of some of the worst RF emitters I have ever experienced, or lack of grounding, or lack of lightening protection with antennas/coax/towers/push up pipes connected to houses that would burn to the ground in a lightening strike. Pay attention to details, as those details may come back to bite you.....or you may get lucky and never have a problem.

[WRPC294]

I know this is a old post but I'm wondering what does the faa and FCC consider a "public airport" is it a commercial airport or a airport that anyone can fly in and out of and now um now tying to be a smart a$$ because to me a public airport would be a airport owned by a public place like a city or county or state and is used for commercial use a private airport would be one that is owned by the city or private person and only used upon request  for a certain type of aircraft like smaller planes and not commercial like 747 I may be wrong. Hope some one can help me out  

[tweiss3]

The FAA has a notice criteria tool: https://oeaaa.faa.gov/oeaaa/external/gisTools/gisAction.jsp?action=showNoNoticeRequiredToolForm

 

I have had to file notice for buildings and even substation power lines before. I have also used that tool to prove buildings are not tall enough to extend into FAA airspace. 

[WRQJ277]

I need some help please ! 

I have a comet 712efc and when I went to put the cable back into pipe something snapped. The little stubby coax caught the out side of tube and I heard a snap.

Checked and didnt really see anything that came loose but when I checked swr it was 11 + !

Anyone have a diagram or some close up pictures of coax stub ? 

The little stub of coax is slid into brass collar but center conductor is not touching anything. The outer sheild is soldered to long coax sheild. 

Any help please send me some pics or a diagram to 8036298971 or email them to Briphillipsonebay@yahoo.com 

Cant use this antenna until I figure the swr problem out.

Thanks in advance ,

WRQJ277

Brian Phillips