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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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  #41 (permalink)  
Old 07-07-2017, 1:46 AM
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Originally Posted by JoninNYC View Post
Regarding RF Exposure and PEP; would a beab antenna negate this due to the fact that the energy is directions and I would be behind the antenna in the appartment.
Most directional antennas have a "front to back ratio". Thats the difference between the amount of RF energy that gets directed off the front of the antenna compared to the energy that comes off the rear of the antenna.

The back side of a Yagi does not have zero RF energy radiated. It's usually a small amount, though.

Unless you are running way too much power, or are in a situation where RF energy may be unwanted (medical implants, interference, etc), then it's something to consider. But in most cases it's not enough to worry about.
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Old 07-07-2017, 9:54 AM
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If I went with a beam antenna, would it need to be pointed exactly to the location of the repeater? How wide is the RF beam? If a particular repeater is to the right of the ntennaor even behind id would it still work?
Maybe.

Depends on the antenna. The specs for a beam antenna will show the vertical and horizontal beamspread in degrees. Generally, Yagis with more elements have tighter beamspreads.

Maybe. If your beam antenna spills enough signal off the side to activate the repeater, then "yes". But, the whole idea of a beam antenna is to concentrate your signal in one direction at the expense of other directions. Generally, Yagis have a fair amount of signal to the rear and very little signal to the sides.
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Old 07-07-2017, 2:21 PM
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Most vertical antennas transmit to the horizon or above-that is one more reason to get the antenna up in the air(the other being range-antenna height is more of a factor than power). Remember you are responsible for RF exposure of others exposed to your station emissions. While you can be in a null of a beam, you are responsible for the 'public' in the main lobe.

I have found if you can't make the repeater with 20-45 watts, more power won't help much. Get the antenna outside and 'in the clear' and you likely won't need much power-most of the satellites are under one watt and recommend 5 watts or less for the uplink- the key is line of sight.

Also the rules state using the minimum power needed to communicate-see 97.313a- not followed as much as it should be.

73
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Old 07-08-2017, 11:20 PM
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Default Mag Mount

if I were to keep things simple for my temporary patio installation and just go with a dual band mag mount, what do I do about the coax? The coax that comes with a ma mount surely won't be long enough, as those are made for the car. I would probably need to add 20 or 25 feet. What the best way to do this with no loss? Also, can anyone recommend a good antenna model with the best gain?
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Old 07-09-2017, 12:40 AM
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Originally Posted by JoninNYC View Post
if I were to keep things simple for my temporary patio installation and just go with a dual band mag mount, what do I do about the coax? The coax that comes with a ma mount surely won't be long enough, as those are made for the car. I would probably need to add 20 or 25 feet. What the best way to do this with no loss? Also, can anyone recommend a good antenna model with the best gain?
No loss? Not possible. All real world transmission line will have some amount of loss. The amount of loss is in direct proportion to it's price, so it really comes down to a budget vs. reality thing.
For 20 - 25 feet, just get a length of pre-terminated LMR-400. It's a good basic cable with reasonable loss numbers:
146MHz, 25 feet: 0.374dB or about 8% of the signal.
446MHz, 25 feet: 0.669dB or about 14% of the signal.
Very reasonable numbers for amateur radio use. You can certainly go with higher spec cable, but the gains will become minimal, and down to the point where you and your radio will not be able to tell the difference.

As for using a magnetic mount as a temporary baseÖ
The issue you'll run into is that -most- antennas will want a ground plane under the antenna. Similar to what the vehicle body would provide. You'd need to add something metal under the antenna to provide that. A big steel sheet, like a pizza pan, cookie sheet, piece of sheet metal, etc. will work just fine.
Or, you can get NMO "base station adapters" that will do the trick, too.

As for antennas, my favorite has always been Larsen. Larsen makes commercial/professional grade antennas. I've got 30 year old antennas still working great long since outlasted the vehicle they were mounted on. They might cost slightly more, but they'll last decades. Stay away from the Tram/Browning Chinese knockoff antennas, and if possible, avoid the amateur radio/hobby grade stuff. Often it's not any cheaper and the build quality is lower.
Something like the NMO-2/70 would be a good choice. Pair that with an NMO magnetic mount and the pizza pan ground plane, and you'll have something that will work well.
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Old 07-09-2017, 2:20 AM
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Why don't you build a simple ground plane? You can make it for next to nothing, and camoflage it on the balcony!
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Old 07-09-2017, 2:26 AM
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Quote:
Originally Posted by mmckenna View Post

Something like the NMO-2/70 would be a good choice. Pair that with an NMO magnetic mount and the pizza pan ground plane, and you'll have something that will work well.

I used a sheet of galvanized steel about 2 sq meters with a mag mount Tram on my fiberglass roof RV. Didnít want to go with a 1/2 or 5/8 wave due to the height. Glued it down with some construction adhesive. Works great with good low SWR. Itís cheaper than a pizza pan, and is actually a mathematically suitable ground plane for 144/440.



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Old 07-09-2017, 12:12 PM
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I just looked at the Larson NMO 2/70 - only 19 inches so certainly low profile!
So some questions I have:

1- My choices seem to be a Mag Mount or a Ken Fong J-Pole. The j-Pole in interesting since it requires no ground and can go in a flower pot. Which would perform better, a J-pole or a Mag mount dual band antenna like the Larson?

2- Someone mentioned a low profile ground plane antenna. I don't have the inkling to built one. The ones I have seen online all have long radials which I was trying to avoid. Am I missing something?

3- NMO magmounts come with 17' of coax at the most. May be just a tad short, I think I need 25.

4- I believe the railing on the balcony is some kind of metal (I haven't moved in yet) - Would the railing work as a ground lane?

5- And finally, how safe would operating on 50 watts be with the antenna on the balcony and people say 15 or 20 feet away inside the apartment? (2 meter and 70cm) I used the PEP calculator and it says its fine, but I just wanted to hear some opinions.


Thanks!!!!
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Old 07-09-2017, 1:11 PM
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Quote:
Originally Posted by JoninNYC View Post
I just looked at the Larson NMO 2/70 - only 19 inches so certainly low profile!
So some questions I have:

1- My choices seem to be a Mag Mount or a Ken Fong J-Pole. The j-Pole in interesting since it requires no ground and can go in a flower pot. Which would perform better, a J-pole or a Mag mount dual band antenna like the Larson?

2- Someone mentioned a low profile ground plane antenna. I don't have the inkling to built one. The ones I have seen online all have long radials which I was trying to avoid. Am I missing something?

3- NMO magmounts come with 17' of coax at the most. May be just a tad short, I think I need 25.

4- I believe the railing on the balcony is some kind of metal (I haven't moved in yet) - Would the railing work as a ground lane?

5- And finally, how safe would operating on 50 watts be with the antenna on the balcony and people say 15 or 20 feet away inside the apartment? (2 meter and 70cm) I used the PEP calculator and it says its fine, but I just wanted to hear some opinions.


Thanks!!!!
Ground planes are very simple antennas. For 2 meters, they are about 17 inches high with 17 inch radials! They can easily be placed in a flower pot on a table, and disguised as plants using artificial shrubs, leaves or flowers! You could even hang your "plant" from the top of the terrace!
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Old 07-09-2017, 5:11 PM
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Quote:
Originally Posted by JoninNYC View Post
I just looked at the Larson NMO 2/70 - only 19 inches so certainly low profile!
So some questions I have:

1- My choices seem to be a Mag Mount or a Ken Fong J-Pole. The j-Pole in interesting since it requires no ground and can go in a flower pot. Which would perform better, a J-pole or a Mag mount dual band antenna like the Larson?
Gain is the same between the two. The 1/4 wave will have a better radiation pattern. The J-poles are a 1/2 wave antenna which will work without the ground plane, but they have a different radiation pattern. Might work just fine. I haven't heard a lot of good things about the Fong antennas.
The J-pole will be twice as tall as the 1/4 wave, so keep that in mind.

Quote:
Originally Posted by JoninNYC View Post
2- Someone mentioned a low profile ground plane antenna. I don't have the inkling to built one. The ones I have seen online all have long radials which I was trying to avoid. Am I missing something?
Nope. The ground radials will be closer to 19 inches long on 2 meters. They are required for a ground plane antenna (at least if you want it to work well)

Quote:
Originally Posted by JoninNYC View Post
3- NMO magmounts come with 17' of coax at the most. May be just a tad short, I think I need 25.
Most do. There's a few options:
If you check with a company like Tessco, you can purchase NMO mounts with longer cable runs. I've seen 25 and 30 foot lengths.
You can also buy some NMO mounts as well as the NMO base kits with an N connector on the bottom. That makes it easy to install the length of cable you need.

Quote:
Originally Posted by JoninNYC View Post
4- I believe the railing on the balcony is some kind of metal (I haven't moved in yet) - Would the railing work as a ground lane?
Partially, however an ideal ground plane will extend in all directions under the antenna. Unless the railing in in an X formation, it's going to be less than ideal. It would work, but it can make the antenna slightly directional. A proper ground plane will give you better performance.

Quote:
Originally Posted by JoninNYC View Post
5- And finally, how safe would operating on 50 watts be with the antenna on the balcony and people say 15 or 20 feet away inside the apartment? (2 meter and 70cm) I used the PEP calculator and it says its fine, but I just wanted to hear some opinions.
It's fine. You don't want long term exposure to high RF fields, but 15 feet away from 50 watts on 2 meters and 70 centimeters. Ideally raise the antenna up higher than your head.
Also, it won't be like you are transmitting all the time, you should be doing a fair amount of listening.
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Old 07-14-2017, 11:40 PM
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I am torn between the Diamond SG7900NMO and the SG7500NMO for the patio (on a mag mount on top of a piece of sheet metal.
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Old 07-15-2017, 3:23 PM
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Forget the Diamond junk over rated antennas, save your money. A Larson NMO 2/70 will work just fine on your "piece of sheet metal" groundplane.

Had one of those Diamond SG antennas on a Sheriff's SAR Expedition, did not work, way too many complaints, changed out to a quarter wave and no more problems.
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Old 07-15-2017, 4:17 PM
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I thought Ed's antennas were also known as a Slim Jim ?
Yup, and Slim Jim's work very well as they radiate at a low angle rather than warming up the clouds.

75Watts! Wow - I can get everything I want to hit with 5 watts and where I used to live with a view over the sea I could occasionally hit a repeater about 50miles away across the bay!

Here's the original article....

Original Slim Jim Antenna Article by F.C. Judd - The 2BCX Slim Jim Antenna - G2BCX
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Old 07-15-2017, 4:41 PM
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Yup, and Slim Jim's work very well as they radiate at a low angle rather than warming up the clouds.


Slim Jims have negative gain. Imagine what you could do with gain!


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Old 07-15-2017, 8:45 PM
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Perhaps if you read the article and looked the polar diagrams you wouldn't make such uninformed statements.

Quote from Wiki...
Quote:
Primarily a dipole, the J-pole antenna exhibits a mostly circular pattern in the H plane with an average free-space gain near 2.2 dBi (0.1 dBd).[5] Measurements and simulation confirm the quarter-wave stub modifies the circular H-plane pattern shape increasing the gain slightly on the side of the J stub element and reducing the gain slightly on the side opposite the J stub element.[5][6] At right angles to the J-stub, the gain is closer to the overall average: about 2.2 dBi (0.1 dBd).[5] The slight increase over a dipole's 2.15 dBi (0 dBd) gain represents the small contribution to the pattern made by the current imbalance on the matching section.[5] The pattern in the E plane reveals a slight elevation of the pattern in the direction of the J element while the pattern opposite the J element is mostly broadside.[6] The net effect of the perturbation caused by quarter-wave stub is an H-plane approximate gain from 1.5 to 2.6 dBi (-0.6 dBd to 0.5 dBd).
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Old 07-15-2017, 9:04 PM
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Perhaps if you read the article and looked the polar diagrams you wouldn't make such uninformed statements.


I read the article. They make no claim of gain characteristics. A j-pole is a folded dipole. If you had no feedline, you would get 0 dBd or 2.1 dBi. Most j-poles radiate down into the feedline creating loss, plus the additional loss in your coax length. All this points to a net negative dBd. A little background information here http://www.jpole-antenna.com/2014/03...ain-explained/


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Old 07-15-2017, 9:22 PM
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A j-pole is a half-wave antenna with a quarter-wave matching stub at the bottom. A 'slim-jim' has another half-wave folded back on itself.
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Old 07-16-2017, 11:32 PM
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Originally Posted by JoninNYC View Post
Oh I understand now. I actually bought a fold up Pole on eBay last year for my HT. I have only used it indoors. I probably could just hang that on that patio. Its rated for 100 watts.
Welcome!
This is actually the same one (N9TAX slim jim) i am using in my attic right now with my Yaesu Ft-8800R. It receives very well. Tx on the other hand is a little lacking but i believe that is the location and repeater distance more than anything else. I have used the same one in my front yard with it strung up in a tree and made a simplex contact at 16 miles on 5 watt vhf. I really like them for their size and they make an awesome backup. I have also seen where people have taken these, installed them inside a piece of PVC and then painted the PVC and used it for vines/plants as a way to hide the fact that its an antenna. Maybe not the "best" way of doing things, but it keeps neighbors from giving you grief if they wanted to have something to complain about.
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