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Midland mxt275 as a s.h.t.f. base station / ground plane question


Ccleveland1226
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I have a mxt275 that I would like to be able to take the head unit out of my truck and into the house to use as a temporary base station if the need arose. It wouldn't be permanent so I don't want to spend alot of cash if not needed. From what I have read online, using a mobile antenna as a base antenna requires a ground plane in order to transmit. Would taking a piece of steel plate to stick a magnet mount antenna (like the little stock mxt275 one) work? Or does the plate have to be grounded to earth, or to dc supply ground?

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It depends on how far you want to talk.  If you only need a half-mile or so, then just stick that mag-mount on top of your refrigerator or washing machine lid.  Yes, a flat plate of steel or sheet metal will work as a ground-plane on the GMRS band. I have had success using a 14" pizza pan.

 

Just don't put a temp installation like this in expecting to talk 5 miles, because there is no way that will happen.

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...using a mobile antenna as a base antenna requires a ground plane in order to transmit. Would taking a piece of steel plate to stick a magnet mount antenna (like the little stock mxt275 one) work? Or does the plate have to be grounded to earth, or to dc supply ground?

For your purposes, the ground will be the braid of the coax cable. You do not need a full circular ground plane, three or four radial wires will do just fine. Typically they would slope downward at around 45°. You might also look at an antenna design that did not require a ground plane. Most UHF gain antennas fall into that category.
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I'm trying to get 2.5 miles across flat elevation but through a densely wooded area. When I use this as a temporary base the antenna will be outdoors and about 10' off the ground

If you are interested in hitting one specific receiver, like a repeater, you might consider a directional antenna if the omni directional antenna doesn’t work for you.
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Aah. In that case I would suggest you buy a radio dedicated to your house for the purpose of talking with your mother. That would allow you to use it on a regular basis, maybe even a nightly check in, and let her get used to the operation of the system. I would also suggest you try with two simple quarter wave antennas. But, prepared to go pick up a couple of yagis. By the way, as long as you are just talking line of sight directly to your mother, you don’t need much in the way of the radio. It doesn’t even need to be repeater capable.

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I would not move a radio or radio parts back and forth between the vehicle and home very often. There is a limit on how long spring strength for connections holds out for, based on total number of times the spring is flexed. You will end up wearing out cables, cords and connectors pretty quickly.

 

Do yourself a favor and get a dedicated radio for the house.

 

As far as antennas go, a beam would be nice... but maybe pricey. There are companies that make screw-on ground radial kits so you could use a cheap mobile omnidirectional antenna.

 

2.5 miles through heavy woods should not be a problem. Yesterday I was talking simplex with my son, mobile to mobile, covering 8 miles of hills and woods with only 2 watts. There was a max of 100 feet in elevation change. Unless you have a ton of RFI, you should be fine.

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I’m currently using an MXT-275 to do exactly what you want to do. I talk to my Mom, who is 6 miles away, and she has an MXT-115. We both are using Ed Fong GMRS antennas in our attic. Very cost effective for what we get. Her setup was near flawless. I had to install ferrite magnets on my feed line just below my antenna to hear her. After the magnets, it works great. We do have a 500’ elevation difference, but live in a fairly dense suburban area.

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I’m currently using an MXT-275 to do exactly what you want to do. I talk to my Mom, who is 6 miles away, and she has an MXT-115. We both are using Ed Fong GMRS antennas in our attic. Very cost effective for what we get. Her setup was near flawless. I had to install ferrite magnets on my feed line just below my antenna to hear her. After the magnets, it works great. We do have a 500’ elevation difference, but live in a fairly dense suburban area.

 

I just saw your posting about an antenna in your attic for the MXT-275.  I have an MXT-275 in my vehicle; like it!   But I was thinking of purchasing a base unit for my office in the Townhouse but it's basically all wood up in the 3rd floor attic.   Do you believe your attic surroundings drastically effect your signal output?  What do you think?   Thanks!

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I just saw your posting about an antenna in your attic for the MXT-275.  I have an MXT-275 in my vehicle; like it!   But I was thinking of purchasing a base unit for my office in the Townhouse but it's basically all wood up in the 3rd floor attic.   Do you believe your attic surroundings drastically effect your signal output?  What do you think?   Thanks!

I have a wood 3rd floor attic on my old house as well.  I do have a UHF antenna up there, and it works very well. - not as good as my outside antenna, but the results are excellent for what it is.  I normally get about 3 miles to my handheld radios, and about 7 miles to my mobile.

 

If you are going to put your antenna in the attic, just be sure you do not have metal shingles on your roof.  ...also keep the antenna a few feet away from any old cast iron drain vent pipes that may be going up through the roof.  PVC plastic pipes don't seem to matter.

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As you point out, anything between an antenna's radiating elements and open air does affect the efficiency of the antenna. Metal pipes and ducts have a greater role in they can create RF shadows or even act as a parasitic radiator that either opposes the intended pattern or will add to the pattern creating an unintended lobe. It's these extra elements that are used to create directional Yagi antennas that shape the radiation pattern and "steer" it into a focused beam.

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In response to your attic question... I installed one in the attic and one outside because I was extremely curious to see the difference. With my home where it is, I don’t see much difference but I’m also on a hill and have line of sight to just about any one I talk to. My advice is to try and see. I am crazy that way as I like to figure out how and why things

work.

If it helps, I recently moved my MXT-275 to my Jeep with a 5/8 wave antenna and from my driveway have to punch through 2 houses before traveling down the hill to my moms house and although we can communicate, it’s very staticky.

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If one uses directional antennas on two "base stations" then they will fall under the "fixed station" GMRS FCC rules, correct? Those rules limit the power on "fixed stations" as opposed to mobile and base stations if I remember right.

 

§95.1767   GMRS transmitting power limits.

(a) 462/467 MHz main channels.

          (1) The transmitter output power of mobile, repeater and base stations must not exceed 50 Watts.

                 (2) The transmitter output power of fixed stations must not exceed 15 Watts.

 

"Fixed" stations are those which have directional antennas (ie, Yagi) that are aimed at each other and communicate only with each other, I believe.

 

§95.303   Definitions.

                Fixed station. A station at a fixed location that directly communicates with other fixed stations only.

 

If you are interested in hitting one specific receiver, like a repeater, you might consider a directional antenna if the omni directional antenna doesn’t work for you.

 

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No... that is not correct. A fixed station is determined by its intended and actual use, not its hardware.

 

An example would be to RF linked repeater controllers talking exclusively to each other.

 

Two stations that are base stations and have high gain antennas are not the same thing, since you can (and likely will) talk to mobile and portable stations as well.

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