Antenna height vs. coax length.

Marchboom

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I will be mounting a discone scanner antenna in the attic. The convenient location is right by the attic entrance. However, there is a place to mount the antenna 10 feet higher but that would require another 25 feet of RG-6 coax (along with the 55 feet already being used). Would the benefit of extra height be offset by the signal loss of the longer coax?

Another quick question. I have a Larsen 2/70 antenna (also in the attic) for my 2 meter/440 radio. Would those add-on ground planes be a good investment?

Thanks
 

mmckenna

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I will be mounting a discone scanner antenna in the attic. The convenient location is right by the attic entrance. However, there is a place to mount the antenna 10 feet higher but that would require another 25 feet of RG-6 coax (along with the 55 feet already being used). Would the benefit of extra height be offset by the signal loss of the longer coax?
More cable = more loss. So you'd need to weight that extra loss against what you'd gain by raising the antenna.
Can you hear everything you need to hear now? Is the signal strong, or weak and scratchy?

Raising the antenna will increase the "distance to horizon", and since most of what you are likely listening to on your scanner is line of sight, there may be some benefit. On the other hand, raising an antenna from 20 to 30 feet is going to net you about 1.2 miles of increase to the visible horizon.

If you do need more range, 10 feet might help a bit, and you can always use a higher grade coaxial cable to overcome the loss. Stepping up to RG-11 or LMR-400 will recover the losses.


Another quick question. I have a Larsen 2/70 antenna (also in the attic) for my 2 meter/440 radio. Would those add-on ground planes be a good investment?

Thanks
Yes, it'll help.
Those are 1/2 wave on VHF. With no ground plane, it's 0dB gain. Add a ground plane, and it will give you about 2.4dB of gain. It'll compress the radiation pattern more towards the horizon, and that can help.

And it doesn't need to be the NMO Base kit. You can get a 38" x 38" piece of sheet metal, pop a 3/4" hole in the center, mount your NMO mount on that, and reinstall your antenna. It'll work just as well, will be cheaper and no one will see it up in your attic.
 

Marchboom

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More cable = more loss. So you'd need to weight that extra loss against what you'd gain by raising the antenna.
Can you hear everything you need to hear now? Is the signal strong, or weak and scratchy?

Raising the antenna will increase the "distance to horizon", and since most of what you are likely listening to on your scanner is line of sight, there may be some benefit. On the other hand, raising an antenna from 20 to 30 feet is going to net you about 1.2 miles of increase to the visible horizon.

If you do need more range, 10 feet might help a bit, and you can always use a higher grade coaxial cable to overcome the loss. Stepping up to RG-11 or LMR-400 will recover the losses.




Yes, it'll help.
Those are 1/2 wave on VHF. With no ground plane, it's 0dB gain. Add a ground plane, and it will give you about 2.4dB of gain. It'll compress the radiation pattern more towards the horizon, and that can help.

And it doesn't need to be the NMO Base kit. You can get a 38" x 38" piece of sheet metal, pop a 3/4" hole in the center, mount your NMO mount on that, and reinstall your antenna. It'll work just as well, will be cheaper and no one will see it up in your attic.
For the Larsen antenna, it's on a mag mount and I want to attach it to a piece of 2" angle iron screwed to an upright 2"X 4". To use the sheet metal method do I put the sheet metal under the mag mount or between the mag mount and the screw-on antenna? (great idea!)
 

prcguy

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A friend of mine, a senior staff scientist for Raytheon did a study once on the trade off's of raising an antenna vs feedline loss. His model was an urban environment with some low rolling hills and distant antennas at different heights like mobile and base stations. He found raising a VHF/UHF antenna is beneficial for every foot you can get, up to the point of 8dB feedline loss, then the tables turn. Type of coax didn't matter and you can go higher with low loss coax over RG-58 but the results were always the same. His exercise proved height is most important for an urban environment in getting over buildings, trees, etc to maximize range.

If you repeat the exercise with the antenna on a tall mountain then the results would be different.
 

mmckenna

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For the Larsen antenna, it's on a mag mount and I want to attach it to a piece of 2" angle iron screwed to an upright 2"X 4". To use the sheet metal method do I put the sheet metal under the mag mount or between the mag mount and the screw-on antenna? (great idea!)
Yes, that'll work. Think of it like putting a mag mount on the roof of a car. The cars roof acts as the ground plane. Doesn't even need to be steel. Steel is only needed for the magnet to stick. You could achieve the same results with a piece of cardboard with aluminum foil on it. You just need a conductive material under the mag mount.
 

wowologist

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Make sure to utilize proper separation (should really be vertical to each other) when you have a transmitter antenna close to a receiving antenna. Your going to blind your rec., and depending on your power out, you could seriously damage your receivers front end.
 

Marchboom

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Whatever I use for the ground plane, aluminum foil, metal rods, etc., does this have to be attached to the braid in the coax as a ground?
 

mmckenna

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Whatever I use for the ground plane, aluminum foil, metal rods, etc., does this have to be attached to the braid in the coax as a ground?
In an ideal situation, yes. That's why permanent mount NMO's work a bit better than a magnetic mount.
But, the magnet mount will provide some capacitive coupling if you just sit the mount on the metal.

I'd not take the mag mount apart to achieve that, but if you wanted to, get the piece of sheet metal (steel, aluminum, mesh, etc) and make the 3/4" hole and install a permanent NMO mount.
 

mmckenna

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My Larsen antenna is 34" tall. What diameter should the ground plane disc be?
You need at least 1/4 wavelength in each direction from the base. For 2 meter use, 19 inches from the center point out to the edge.
For UHF, 7 inches or more. So a piece of metal 38 inches square with your hole in the center is just fine.
 

WB9YBM

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I will be mounting a discone scanner antenna in the attic. The convenient location is right by the attic entrance. However, there is a place to mount the antenna 10 feet higher but that would require another 25 feet of RG-6 coax (along with the 55 feet already being used). Would the benefit of extra height be offset by the signal loss of the longer coax?

Are you going to upgrade the coax to lower-loss stuff in the process? (You might add a pre-amp at the antenna in either case). Ancient antenna proverb: "Height is Might." Not only do you see further over the horizon but you also get above the ground clutter.
 

Marchboom

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I would like to install better coax but the coax runs thru electrical conduit and has several bends that prevent a larger line. I do have an in-line pre-amp that I'll be installing. It's an old Radio Shack 10dB coaxial unit but worked very well last time I use it. I'm sure there are better ones out there now. My house is on a hill that is probably higher than other houses for probably 30 miles around. The only thing that would be in the way of a line of sight are pine trees.
 

ab3a

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Using a preamp does mitigate some of the coaxial loss, but it could also introduce other problems if it overloads. If you're in a relatively rural area with no nearby broadcast stations, you'll probably be okay.

Common sources of overload are nearby FM and TV broadcast stations. One solution is to get an FM band notch filter. An example is below. I have no affiliation to this site, but I have purchased things from them in the past and liked the products. I have not tried this specific product because I live far enough away from most transmitting sites that I do not need one.

You should place the filter in front of the preamp. This will prevent the FM broadcasters from overloading your preamp.

Another solution is to just get a better preamp. Technology has improved since the days when Radioshack used to sell such things.

Good luck!
 

wowologist

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Kind of a too each his own question, but can I ask why your using a mobile antenna in your attic?

I would go more towards a j-pole or even a dual band loop. ( Arrow Antenna J Poles 2m 146 70cm 440 OSJ ) (no affiliation or gain given to me by any site or product suggestion) ( https://www.amazon.com/Dual-Band-400-470MHz-Horizontal-GABIL-GRA-YG1443/dp/B07Y4S5BMF ) if your mechanically inclined both of these antennas are actually very easy to make, very robust and the parts are available at your local big box hardware store.

I have made numerous of both of these for myself and other hams just starting out. Ther is also the "slim-jim" version of a j-pole. ( Authentic N9TAX VHF/UHF Slim Jim J-Pole Dual Band 2m 70cm Antenna jpole | eBay ) Any of these will solidly outperform a mobile antenna being used as a compromise in a non-mobile mounting position. And btw the ladder line slim-jim can be handmade for literally >6$
 

BushDoctor

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Strasburg, Va
I will be mounting a discone scanner antenna in the attic. The convenient location is right by the attic entrance. However, there is a place to mount the antenna 10 feet higher but that would require another 25 feet of RG-6 coax (along with the 55 feet already being used). Would the benefit of extra height be offset by the signal loss of the longer coax?

Another quick question. I have a Larsen 2/70 antenna (also in the attic) for my 2 meter/440 radio. Would those add-on ground planes be a good investment?

Thanks
My high band (2 meters) ground plane antenna is at 708 feet I raised it almost 10 feet higher (88 inches higher) and got about .3 mile further range to a mobile that was 31 miles away on 708 feet and raising it 88 inches gave it an additional .3 of a mile. The 31 mile point was on a ridge and the ,3 was further along the highway but at the same level as the ridge(level road) I have mountains all around but in that direction just smaller hills like i am on. Using a 3 db gain on mobile made the signal a little quieter on background noise
 

Marchboom

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Kind of a too each his own question, but can I ask why your using a mobile antenna in your attic?

I would go more towards a j-pole or even a dual band loop. ( Arrow Antenna J Poles 2m 146 70cm 440 OSJ ) (no affiliation or gain given to me by any site or product suggestion) ( https://www.amazon.com/Dual-Band-400-470MHz-Horizontal-GABIL-GRA-YG1443/dp/B07Y4S5BMF ) if your mechanically inclined both of these antennas are actually very easy to make, very robust and the parts are available at your local big box hardware store.

I have made numerous of both of these for myself and other hams just starting out. Ther is also the "slim-jim" version of a j-pole. ( Authentic N9TAX VHF/UHF Slim Jim J-Pole Dual Band 2m 70cm Antenna jpole | eBay ) Any of these will solidly outperform a mobile antenna being used as a compromise in a non-mobile mounting position. And btw the ladder line slim-jim can be handmade for literally >6$
I obtained my No-Code Tech lic in '94 and bought a Kenwood TH-78A HT and a Comet mobile antenna which I used on my car. Recently a friend gave me a moblie Kenwood 2 meter/440 radio, a few HTs, a Mirage linear amp for 2 meters and a Larsen 2/70 antenna with a mag mount. So I have decided to put it all together with what I have to see If I really want to get more involved with ham radio. If I do then I'll upgrade my lic. and the equipment. But for now I'm just using what I have available.

I made a ladder antenna years ago so maybe I'll try it out. I assume that I'll need a SWR meter to see what antenna works the best.
 
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