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Author Topic: antenna height  (Read 1392 times)
WDAWG
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Posts: 3




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« on: February 22, 2007, 05:56:41 PM »

Hello,

   I will be taking the technicians exam in the next few days.  I am in the process of building my base station.  Starting out I would like to work simplex and local repeaters on 2 m and 70 cm.  I’ve almost settled on the  Commet GP-9N dual band antenna.  I’m not sure how high I should try and mount this antenna.  How high is high enough?  Any good (recommended) references on antenna height and propagation?  Any help would be appreciated.

 

Over,
 
TBD_03_TBD    
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K9KJM
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Posts: 2415




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« Reply #1 on: February 23, 2007, 12:22:06 AM »

They'll all tell you the same thing: "The higher the better".............
It would be nice to get the antenna above local obstructions, But just do the best you can.
The coax feedline is VERY important at VHF and UHF.
Times LMR-400 is the good stuff to use for lengths less than 70 or so feet. Keep the coax as short as possible.
Good luck on the exam, And welcome to the hobby!
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N0RZT
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« Reply #2 on: February 23, 2007, 06:39:10 AM »

It depends on how local "local" is.  If you can hit the repeaters with the ducky on your HT, then your base station antenna doesn't need to be at 50ft.  If "local" is "within 50 miles" then you may need to put the antenna on the roof of your house.

Take care,
Chris
N0RZT/8
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WB2WIK
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« Reply #3 on: February 23, 2007, 08:28:50 AM »

When working repeaters, the repeaters themselves do most of the work.  Well sited repeaters can often cover 100 mile radius, even for mobile users, so this is not a demanding application for a "home station."
 
Simplex is very different.  Your own station determines performance.  Getting the antenna up and above all local obstructions including houses, utility lines and trees makes a big difference.  After you're above all of those things, small changes in antenna height don't matter much, but going up 10' at a time or so does help extend your working radius.  As you increase your working radius, you increase the number of stations you can contact by a square function, since area is a square function of radius; that is, doubling your radius increases the number of stations you can contact by a factor of four.  Tripling your radius increases it by nine.   Quadrupling your radius increases it by 16.

This is the reason height above ground for VHF-UHF antennas really is important, for general-purpose operations.

WB2WIK/6
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WDAWG
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« Reply #4 on: February 23, 2007, 07:00:13 PM »

Thanks for the responses.  I will put the antenna on top of my 2 story house.      This will clear all local obstacles. Increasing the height over time should be simple.


 For cabling I will use Andrew’s LDF5-50A.  This stuff has 0.808 dB/100 ft of loss at 450 MHz.   I snatched up 275 feet of the cable at a yard sale for $5.00 cash.  I’ve stored the cable for 1.5 years, and at last I am building the station and taking the test (tomorrow).  My wife laughed at the cable but tomorrow she will take the test as well.
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K7SU
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« Reply #5 on: February 23, 2007, 09:34:33 PM »

Seems a paradox..."get it as high as you can but keep the coax as short as possible".....but...it's true!

:>)

Kelly
K7SU
www.K7SU.4t.com
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N0RZT
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Posts: 104




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« Reply #6 on: February 24, 2007, 02:12:15 PM »

Well, you can keep the coax run short as long as you place the radio on the top-most floor - but keep the grounding cable as short as possible ;-)

I remember seeing a flea-power ATV setup that had the radio on the mast next to the antenna.
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AA4PB
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« Reply #7 on: February 24, 2007, 05:13:55 PM »

Its not unusual to find a receive preamp up on the mast by the antenna, especially on the higher frequencies where cable loss is higher. The transmitter is most often on the ground because you can always make up for cable loss with more power output.
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WB2WIK
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« Reply #8 on: February 24, 2007, 05:45:11 PM »

>RE: antenna height  Reply  
by K7SU on February 23, 2007  Mail this to a friend!  
Seems a paradox..."get it as high as you can but keep the coax as short as possible".....but...it's true!<

::I don't believe in that theory, at all.  Who cares how long the coax is, if it's all running vertically?  The only coax that adds loss to a system is the cable that runs horizontally.

Seriously.

WB2WIK/6
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K9KJM
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« Reply #9 on: February 25, 2007, 12:41:26 AM »

Andrew LDF5-50A is indeed the "Good stuff" for VHF/UHF use!
Connectors are available used on Ebay or at swapfests if you need them, Expect to pay around 20 bucks each for nice used connectors.
No special tools are needed to install them either, A good hacksaw, small file, A few wrenches, etc.
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WDAWG
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Posts: 3




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« Reply #10 on: February 25, 2007, 01:47:11 PM »

I passed the test.  Now I’m waiting to be listed.

I put the antenna on my front porch at an angle of ~50 degrees off zenith. With 15 feet of cable I was able to listen to some repeaters about 50 miles away.  Nothing heard on the   simplex bands.  What are the common channels?    I don’t want to install the antenna until I can measure the VSWR to insure proper operation across both the 144 MHz and 70 cm bands.  Additionally I’m not climbing on the roof until the snow melts.  My transceiver (IC-706MKIIG) will not measure VSWR on these bands.  Finding this out last night was annoying.  Now I need an SWR meter.  The connectors for the cable are expensive, ebay here I come.  This is getting expensive.  



I’m hoping to not install an amplifier on the mast.  A pre-amp could cause a near-far problem for simplex.  If horizontal cables add no loss, then maybe a zigzag cable will attenuate big signals?      

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KB3OLK
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« Reply #11 on: February 25, 2007, 05:46:58 PM »

For simplex, start with 146.52 (National Simplex Calling Frequency).

Also, depending on where you live, the regional band plan can give you some insite.  This link can get you to your local band plan: http://www.arrl.org/nfcc/coordinators.htm

Your local amateur radio club might have a web page that indicates what frequencies are commonly used in your area.

Best idea: relax and work the repeaters as soon as your license shows up in the ULS!
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WB2WIK
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Posts: 20601




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« Reply #12 on: February 26, 2007, 10:07:04 AM »

>RE: antenna height       Reply
by WDAWG on February 25, 2007    Mail this to a friend!
I�m hoping to not install an amplifier on the mast. A pre-amp could cause a near-far problem for simplex. If horizontal cables add no loss, then maybe a zigzag cable will attenuate big signals?<

::A remote preamp, for most, isn't a good idea because they are prone to failure and the ones that bypass themselves for transmitting are expensive.  Good cable is much cheaper.

The "no loss from horizontal cables" was a tongue-in-cheek comment!  What I meant was, if all your coax runs "vertically," that means your antenna is as high as your coax is long, which is a very good thing.   Therefore, the loss in that cable will always be canceled by the height gain of the antenna.

Cables that are run horizontally don't provide any height gain.

WB2WIK/6
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N0EW
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« Reply #13 on: March 28, 2007, 10:12:13 AM »

http://www.arrl.org/FandES/field/club/clubsearch.phtml

If you point your browser to the above link you'll be able to search for ARRL amateur radio clubs. There are also clubs not associated with the ARRL.

If there is a good club in your area you should be able to find someone with the various meters and other odds and ends needed to help you set up your station. That'll both save money in the short-term and perhaps present you with an "Elmer" (coach).

Or maybe not. Clubs don't exist everywhere, and not all of them have knowledgeable people ready to help usher in the new guys.

But you'll only know if you look.

When evaluating another's ability to help, I always first determine whether they display self-knowledge of what I call the Three Levels of Knowledge:

1. What I know I know.

2. What I think I know.

3. What I know I _don't_ know.

Beware the one telling you they know everything about everything. No one does. Those that profess to are unreliable, and you'll have to double-check every single "fact" that drops out of their mouth.

We are an extremely diverse group. Students to physicist; retired to truck drivers. Given some time you are certain to find a number of kindred spirits. So don't give up if the first bunch you meet aren't your cup of tea.

Welcome to the service / hobby. I hope you find it a rewarding experience for years to come!

Erik n0ew
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N0EW
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« Reply #14 on: March 28, 2007, 10:20:21 AM »

http://www.arrl.org/news/stories/2007/03/20/100/?nc=1

The above article discusses how the new FCC rule changes (dropping the CW / Morse code test) have affected your new Tech class privilages on HF.

I haven't closely followed this topic because it doesn't affect me, but I did browse this article. It appears to have some interesting and useful information.

73 Erik n0ew
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