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Author Topic: 2m Antenna height and distance  (Read 2326 times)
ONAIR
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Posts: 1747




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« on: January 08, 2008, 10:17:26 PM »

   Best to go as high as possible.  Use LMR-400 for long runs.
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K9KJM
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Posts: 2415




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« Reply #1 on: January 08, 2008, 10:32:09 PM »

Yes. Your antenna is too low and too far away.
Try keep the coax feedline as short as possible, And get the antenna as high as possible.

Rule of thumb for VHF/UHF coax lengths:
RG-58, RG-8X types, 20-25 or so feet.
RG-213 types, 40 or so feet.
LMR-400, Up to around 75 or so feet.
For any longer runs of coax you really need to get some good used Heliax, Or some "spool ends" of surplus cable TV trunkline, OR consider "remote" operation by putting the radio part of most modern day mobil type radios right at the base of the antenna, And mounting the control head at your operating position. Interconnect the two with something like CAT 5 wire, Thereby keeping the coax run nice and short.

Sure, You could run lots longer lengths than above, Lots of hams do. AND they also HAVE NO IDEA of how many signals they are MISSING out on!!!!!
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GHDUTCH
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Posts: 8




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« Reply #2 on: January 08, 2008, 04:29:44 AM »

New station, put up 2m Diamond x0a base vertical antenna, on a 10 ft pole, with a 100ft coax cable back to station.  Have the antenna attached to a 4x4 fence post which is part of my privacy fence which has rather tall trees on the other side.

 Reading posting about atennas, is mine to low and far from my station [to long of a coax cable run]HuhHuh

Thanks in advance for any input.
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GHDUTCH
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« Reply #3 on: January 09, 2008, 05:07:19 AM »

I see what your saying about a short distance away, but does that mean I should get a shorter cable, or can you just role up the extra length and secure it, or do you need to cut and attach a connector at that length?
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WB2WIK
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Posts: 20633




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« Reply #4 on: January 09, 2008, 11:07:36 AM »

Height makes a lot more difference than line loss, usually.

Rule of thumb: No amount of coaxial cable is "too long" or "too much" if the cable is running vertically.

That's because if all the cable is "vertical," it's the amount you need to reach your antenna.

Coaxial cable run *horizontally* is the cable that wastes signal.

Yes, the only way to reduce line loss is to make the cable shorter, which of course means cutting it and re-installing the connector.  Coiling it up doesn't do anything except make something to trip over.

WB2WIK/6
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K7PEH
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Posts: 1124




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« Reply #5 on: January 11, 2008, 09:00:19 AM »

WB2WIK says "That's because if all the cable is "vertical," it's the amount you need to reach your antenna. "


But, that is indeed the real problem with antennas, isn't it.  I mean, if the cable is vertical then the energy of your signal is climbing out of a gravitational field and as a result it actually loses more energy then if the cable were horizontal (or, rather, along a fixed gravitational potential sphere).

You can actually calculate the loss of energy due to the gravitational field exposed by the earth.  This is the very same phenomena as a black hole.  That is, a super strong gravitational field affects the radiation so much that it never makes it out of the presence if the field as defined by the event horizon of the black hole.  But, you don't need a black hole to cause a loss of energy.

The worst part about this is it is not just the transmission line that is affected but also the radiated energy from the antenna.  The net result is that there is a frequency shift to lower frequencies (less energy).  This could be called the HF shift of VHF (sort of like saying "gravitational red shift").  I am certain that this could be the reason that George, KD7YVV, is always off frequency when we arrange a 146.52 simplex contact.

There is no real good solution to this problem though unless of course you can find some dark energy.  Dark energy is opposite of gravitation.    The gravitational field just sucks the life out of my radio signals but dark energy would give it a boost -- sort of like after burners being switched on.  The boost will result in a blue shift or an increase is frequency.

I have always wanted to operate a 10 GHz station.  I wonder if I could use dark energy as the transverter to my 144 MHz signal.  Now, all I need to do is find out who sells "dark energy".

Calculating gravitational red shift:  You can calculate the shift in frequency (or, change of wavelength) using a simple little formula which I admit is kind of hard to write limited as such on the web.  But, let's call the shift to wavelength "delta-lambda" and the wavelength is "lambda".  Then, the ratio of "delta-lambda" divided by "lambda" is equal to the ratio of "gravitational-acceleration" times "height" divided by "speed of light squared".  You can find more information on this calculation and its derivation in the book "An Introduction to General Relativity, Spacetime, and Gravity" by Sean M. Carroll, published by Addison-Wesly (pages 52 through 54).
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WB2WIK
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« Reply #6 on: January 12, 2008, 02:09:16 PM »

K7PEH has some good points.

However I overcome the gravitational effects for the most part by simply using dB grease.  It's most easily applied to hollow-core transmission lines but can work even with conventional coaxial cables if you use a high pressure gun and specialty connector to apply from one end.

As is commonly heard at most large antenna installations, "Don't forget the dB grease!"

WB2WIK/6
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K9KJM
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Posts: 2415




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« Reply #7 on: January 12, 2008, 11:30:00 PM »

No big secret. Andrew Corp. Supplies a small plastic pouch of that grease with every Heliax connector!
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KB3MMX
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Posts: 132




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« Reply #8 on: January 20, 2008, 03:33:51 PM »

Go High and go Far as they say. 10' AGL is pretty low for VHF and UHF useage.

 I do disagree with the statement that the limit for LMR400 be 75' , this is untrue.
  You can make use of LMR400 up to twice that distance without significant losses.
 If you want to see some extreme losses, check out RG8U and also worse yet, look at RG8X, it is horrible.
 
Anyway, If you need less loss than LMR400, then you should look at the LMR600 or Andrews Cinta 600 cables.
 They are extremely competitive to "Hard Line" in loss and a fraction of the cost!
 Try running some loss calculatins and you'll see what I mean.

http://www.ocarc.ca/coax.htm

73,
    Chuck
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K9KJM
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Posts: 2415




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« Reply #9 on: January 21, 2008, 03:18:05 AM »

For weak signal use, About 75 or so feet of LMR 400 is IT!  Sure, You can run it longer. You will just end up with a crappy station! If you don't care how poorly your station functions compared to someone who uses low loss Heliax, Go for it!
Lots of people use RG-8X for VHF and UHF too, AND NEVER KNOW what signals they are missing out on!
There is nothing wrong with LMR 600, EXCEPT the very high cost of the coax AND it's connectors.
You can buy good used "take off" 7/8" Andrew Heliax complete with connectors from most every ham swapfest, Or from a local two way radio dealer who works on commercial tower sites LOTS cheaper than new LMR 600 would cost.
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KB3MMX
Member

Posts: 132




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« Reply #10 on: January 21, 2008, 05:14:53 AM »

Just try the calculator, and you'll see that you can go longer than 75' without huge losses.Where you really start seein the spread is on 70Cm with length.

 Also, check losses of LMR600 VS Price and you'll find that new to new it beats the tar out of Heliax "hard line", up to 7/8".
 Flat out, for the price the LMR smokes the hard line.
 If you want to compare used hard line to LMR, the LMR will still be cheaper and you can get some HUGE LMR cables, easily forthe price of ultra expensive hard line.
 Sure, you can buy either one used for much less but the LMR will always cost less.
 **I'm not knocking the performance of larger diameter hard line at all, but it isn't worth the money unless... you are using 7/8" or bigger. The 1/2" and 3/8" are a total waste of money compared to LMR cables....
  You are totally right about some people using RG8X, and I know some people locally that do so and they will never know what they are missing because it is all lost in the coax before they could ever hear it!
 Sometimes trying to explain that is like beating your head against a wall...LOL

 One thing is for sure, if you go the $$, go for the biggest and least lossy cable you can put up, LOSS is the ENEMY on any band!!

 Take Care,
 
           Chuck
           KB3MMX
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K9KJM
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Posts: 2415




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« Reply #11 on: January 22, 2008, 01:47:49 AM »

Going price for ANDREW 7/8" Heliax, LDF5-50A complete with connectors is around $1.00 per foot. A BUCK A FOOT!     At most all ham swapfests, Local dealers, Tower construction companies, etc. (Do note that it is difficult to ship 7/8" heliax....)
This size is usually a pick up only item.
While an apples to oranges comparison, Good used Andrew 7/8" Heliax BEATS the HECK out of buying higher priced LMR series larger coax, Along with the high priced connectors!  
Why anyone would want to pay more, For less quality is beyond me!
I have "Been there and done this both ways" A number of times.
AND nowadays, With the ongoing upgrades to cellular phone tower sites, 1 1/4" and 1 5/8" Heliax along with the connectors is coming on the market for about the same surplus price! So if you need to run VHF/UHF a long distance, Check out some of this really low loss Heliax!
(Yes, 30 YEARS AGO When I started to buy and use the 7/8" Andrew Heliax for VHF/UHF it was MUCH more difficult to find, And cost a lot more than it does nowadays. Check the price NOWADAYS on this great deal for low loss VHF/UHF transmission line! Apparently the old law of supply and demand caught up with used Heliax, And the used prices have come WAY down on the coax and connectors!)
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NA0AA
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Posts: 1042




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« Reply #12 on: January 22, 2008, 01:13:19 PM »

The only problem I have with using Heliax is that my antennas are all considered temporary and intentionally so.

IF I had a rigid tower installation where I never needed to make changes to the feedline runs, yea, I'd go the extra work of running it.  But it's hard to feed heliax up a push-up pole, unless you have a few people to help you. <G>

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