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900Mhz feedline and antennas
01-19-2018, 11:01 AM
Post: #1
900Mhz feedline and antennas
Part 1

Speaking only in generalities, us amateur radio types almost always try and do things as cheeply as possible. While this is just fine for HF antennas and feedline it does not always play on the upper bands. Here is an attenuation chart from QRZNow.com for the most popular coax used by hams. Some of the values are scary!

[Image: COAX-LOSS.jpg]

A more detailed sheet for RG type coax is shown here.

[Image: RG-Atten.gif]

And here is the LMR attenuation for 900Mhz.

[Image: low-loss_chart.jpg]


Curt - NA7CS, old-school Advanced Class.
If I wanted to be an appliance operator, I would be working in a kitchen.
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01-19-2018, 11:03 AM
Post: #2
RE: 900Mhz feedline and antennas
Part 2

By now you should have an indication that smaller and cheaper in the upper bands is not the way to go. The signal loss in RG-58 is so massive that it should be avoided whenever possible. In mobile/special circumstances it can be and is used but in lengths less than 17ft. At 900Mhz you loose approximately 2.637 dB with a 17ft run!!!

Take that in for a moment, a loss of 2.637 dB means your radio just lost almost 1/2 its power out and signal in. And DO NOT USE Diamond mount(s) with the RG-174 jumper. You will loose 0.318 dB PER FOOT. This means the 4 ft jumper alone gives you 1.271 dB of loss. When you add in the coax your have a feedline loss of 3.908 dB.

If you must go small, LMR240 (approx 1/4 in diameter) might be the way to go. While still not the best you have a more acceptable loss with a 17ft run of approximately 1.298 dB. At 25 ft you have 1.909 dB loss with a 1:1 SWR. Link for custom cables. The Antenna Farm

Here is a coax cable loss calculator you can use, and the same one I used for the values here. qsl.net coax calculator

While essential in setting up your 900Mhz station, even the best antenna in the world will not perform as designed with bad feedline.

As some of you may have noticed, some companies charge more for 900Mhz antennas than the lower bands. However you can still use homebrew or lower cost antennas. Providing you do not scrimp on the feedline.

For example, a 3dB mobile antenna with 17ft of RG58 is basically a unity gain antenna as the antenna gain nullifies the coax loss. Change the antenna to a 5 dB elevated feed antenna and you now have a little gain on your system. Just make sure you have a proper ground image.

While being far from ideal, this will at least get you on the air. As an example, I had a QSO on 900 Mhz simplex (927.500 Mhz) with Ford (N5FW), and Chris (W7CGC) over a distance of approximately 15 miles. Chris and I were mobile and Ford was at his QTH. All three of us had 3dB gain mobile antennas, however Ford's was slightly different.

Ford's antenna was an NMO mount with 17ft of RG58 mounted on a pizza pan which provided a proper ground image at 900Mhz. The antenna was approximately 12ft above ground level. With this antenna Ford's signal was nearly full quieting. Chris' however was not so fortunate due to his location in a community of stucco (wire mesh) houses. Chris went from nearly full quieting, to non existent.

Curt - NA7CS, old-school Advanced Class.
If I wanted to be an appliance operator, I would be working in a kitchen.
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01-19-2018, 11:04 AM
Post: #3
RE: 900Mhz feedline and antennas
Part 3

900Mhz is a different breed of spectrum with some interesting propagation characteristics. Some beneficial and some not so much. Here is a basic list of propagation factors. (credit to Summit Technical Media, LLC)

Free-space path loss — Solely distance-related; loss vs. distance is 1/d3 (d3 is distance in miles, cubed) in the near-field, 1/d2 (d is distance in miles, squared) in the far-field. The best way to describe this is a follows. "All radio signals have their field intensity reduced with increasing distance, as the wave front spreads out like the surface of an inflating balloon."

Absorption — Atmospheric absorption a relatively small factor; water vapor effects increase with frequency; vegetation and structures (walls, windows, etc.) have significant effects. "Absorption types are part of the wireless environment. These mechanisms include the attenuation due to lossy dielectrics in structures, such as walls, windows, doors, etc."

Natural and man-made noise — Natural noise is very small; man-made noise varies with location and type of environment (e.g. residential vs. commercial).

"At microwave frequencies, natural noise sources are trivial. Atmospheric noise is predominant at lower frequencies; solar and galactic noise is low-level and further attenuated by the atmosphere. This is not the case for man-made noise sources."

"Noise is usually considered to be random and wideband. Many electrically-operated devices have the potential to create wideband noise that reaches to the GHz range. Any “spark” or fast rise time voltage transition generates unwanted signals over a wide frequency range. Although usually small, the proximity of these devices to our wireless equipment can result in detectable, or even interfering noise levels"

"Finally, all the discrete-frequency low-level signals from clock oscillators, leakage and other lowlevel artifacts of normally-operating high frequency circuitry can add up to a measurable total power, with noise-like wideband characteristics."

Reflection, refraction and multipath — Effects increase with frequency due to shorter wavelengths; time-delay effects require compensation in signal processing. " "Signals are reflected from the ground, buildings and other objects. At distinct edges (e.g. corners of buildings) the signals will be diffracted, and variations in the atmosphere cause refraction. In other words, the transmitted signal is scattered, bent and bounced before it reaches the receiver."

The Mobile Environment
At the receive antenna, the direct signal and these modified signals are summed, which can result in either reinforcement or cancellation

WaveRider Communications has studies loss at microwave frequencies and has published their results.

Propagation Characteristics:
Free Space Loss – the loss due to radio energy passing through air with a clear Fresnel zone. Generally, a 900 Mhz signal travels through air with much less loss than does a higher frequency signal.

............................900MHz 2.4GHz 5.8GHz
Example – 3 miles: 105.2 dB 113.7 dB 121.3 dB

Cable Loss – the loss expected due to radio energy passing through RF cables.

Examples:.............900MHz 2.4GHz 5.8GHz
150 feet, LMR400 5.9 dB 10.0 dB 16.1 dB
150 feet, LMR900 2.6 dB 4.4 dB 7.2 dB
150 feet, LMR1200 1.9 dB 3.4 dB 5.6 dB

Vegetation Loss – the loss expected due to radio energy being absorbed by the moisture content of the vegetation. This loss will vary for every situation, however the typical tree loss expected at different frequencies has been estimated by the International Telecommunication Union.

...................................900MHz 2.4GHz 5.8GHz
Example – 150 feet of trees 9 dB 25 dB 60 dB

Wall and Glass Loss – the loss expected due to radio energy being absorbed while passing through the walls andwindows of a building. The amount of absorption varies depending on the construction materials and thickness of thematerial. Generally more energy is absorbed at the higher frequencies.

Curt - NA7CS, old-school Advanced Class.
If I wanted to be an appliance operator, I would be working in a kitchen.
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01-20-2018, 09:36 PM
Post: #4
RE: 900Mhz feedline and antennas
I ordered a couple of these 25ft jumpers:

https://smile.amazon.com/gp/product/B0716P5ZCB


There is also a 75ft jumper with n-male on both ends:

https://smile.amazon.com/dp/B0711HL4TL


Altelix claims a 900 MHz attenuation of 3.9dB per 100ft with their AX400:

http://docs.altelix.com/datasheets/DS_AX400.pdf
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01-21-2018, 05:03 PM
Post: #5
RE: 900Mhz feedline and antennas
I just ordered 50-feet of LMR 400. Once my N-connector to mini UHF connector gets here, I will be good to go (after I get the radio programmed).

[Image: mara-sig.jpg] [Image: dxcc.jpg] [Image: WAC.jpg] [Image: was.jpg] [Image: skywarn.jpg] [Image: vec.jpg] [Image: QRZawards-sm.jpg] [Image: ARRL-Certificates%20-%20sm.jpg]

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01-22-2018, 07:16 PM
Post: #6
RE: 900Mhz feedline and antennas
Thank you Curt for your input in this new frequency endeavor. I think this going to be a lot of fun. As I mentioned on the air I have two of the GTX
radios and Tim, K7OB has one waiting to be programmed. hope to have some antennas soon. 73 de N7CX.
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