external Antena with moteino

[captcha]

The 0.66 and 0.73 is the velocity factor and is greatly dependent on the dielectric material. When you cut a short bit off the outer black insulation and fold back some of the braid, is the material immediately around the centre conductor a hard or soft material? If it's foamy, the velocity factor for that cable could be 0.73. If it's hard-ish semi-opaque material it's probably polyethylene (PE) and your velocity factor will probably close to 0.66.

So for 915MHz a full wavelength is 32.78cm. A half wavelength is 16.39. Compensated for velocity factor you'll arrive at 10.82cm. So use multiples of 10.82cm. If your antenna is perfectly matched to the radio and coax you'll lose about 2dB over a 12 feet length of cable.

Can you reword your last paragraph? Not sure I'm getting it.. 

[d00m178]

Thank you!
and sorry for my bad English.

is greatly dependent on the dielectric material.

sure, I've read already about it and seems to understand this fact.

here are couple of photos of my cable.

details of my cable described here (only russian):
http://www.radiolab.ru/ru/3/1106648468/1152529320/1152610930.php

I can see than dielectric material is FTE.

Regarding velocity factor - probably I made a mistake. This document states that parameter with name "Коэффициент укорочения" (Coefficient of shortening) is 1.28.
I googled about this parameter and seems it means velocity factor. So I decided use this value in calculation.
But today I read carefully wiki (in russian again): https://ru.wikipedia.org/wiki/%D0%9A%D0%BE%D1%8D%D1%84%D1%84%D0%B8%D1%86%D0%B8%D0%B5%D0%BD%D1%82_%D1%83%D0%BA%D0%BE%D1%80%D0%BE%D1%87%D0%B5%D0%BD%D0%B8%D1%8F

and there stated this info:

В зарубежных источниках вместо коэффициента укорочения используется обратная ему величина — коэффициент замедления (Velocity of Propagation, Velocity Factor), выражаемая в процентах.

Here it is said that Coefficient of shortening is inverse value of velocity factor.
So seems it is my bad... I should not to use 1.28 value.
Instead I need to use 0.73 as dielectric material is FTE.
so using online calculator

For coax with Vf = 0.73, 1/2 wavelength = 11.97 cm, 0.393 ft & 4.71 in

11.97*33 == 395.01 cm

Right?

Also I have to install connectors on this cable.

On one side it should be RP-SMA Male  - it connected to such pigtail:


other side of pigtail should be connected to UFL connector on Moteino.
length of pigtail is 26cm from end to end.

On second side will be N-type connector


this connector shoul be connected to my GP antenna.

GP antenna also based on N type - 

so finally, I suppose I have to deduct length of pigtail (26cm) from calculated 395cm == 367 cm

and this 367cm - it is length from the end with RP-SMA (which will be connected to pigtail) - to the point on antenna N type connector that should be already connected to N connector on cable on other side.
and if I connect pigtail it should be 395 cm total.

Or 367cm should be without connected antenna connector? I doubt because on this connector there is part where I should solder vertical copper wire (82mm) and this part has length about 5mm.
Also I'm not sure should I  consider and add this 82mm to total length of cable.. As far I know length of feedline should measure from start point (in my case it is end of pigtail ) and to end point (I suppose in my case it is end of N type antenna connector which connected to N connector on cable)

Please advice for this situation.
Sorry for long read...

As for my GP antenna:  I meant that I build GP antenna, and for it I need 5 pieces of copper wires.
One wire vertical. and 4 wires - on the sides.
And for 915Mhz I need length == 82mm for every wire - it is 1/4 wavelength.
Right?
So my question was - can I use my GP antenna with 82mm wires with cable length of which was calculated with 1/2 wavelengh value. Just want to be sure that it is correct.
Or maybe I need create new antenna - with wires with length equal to 1/2 wavelength == 82*2 == 164mm?

[captcha]

Thank you for the extra pictures and information on what you're attempting to do.

It seems you're not just making a connecting feedline from a piece of coax but also making your own 1/4-wave vertical antenna.

Do you have any measuring equipment to test your antenna system when you've built it? If not, it may be easier to just order one online.

[d00m178]

Yes, I want to create maximum effective antenna system, because I need transmit/receive data in very difficult conditions on long distances (1km) - mountains relief with a lot of radio shadows.
now I can send/receive data from node on direct view from 5km.

As for measuring equipment - I suppose it will cost a lot of money. My budget not allowing me to buy such devices, unfortunately.
Do I really need it?

I supposed if I create correct feedline (I mean - correct calculate length of cable with all the connectors and pigtail) and create simple GP antenna - it should work better that just piece of wire as antenna.

[captcha]

Interesting challenge! :-)

If you want to create the maximum effective antenna system I would highly recommend you have a look at relatively cheap antennas on eBay. They are by no means perfect, and they would still require some proper setup (ground plane, feedline, orientation) but I think you will have one less thing to worry about. Especially if you don't have a means to test your antenna system once you've built it. I'd like to spare you the frustration in case things don't work as expected.

Btw, that piece of wire can be amazingly effective because there is no feedline that could cause impedance mismatches.

Are the communicating stations fixed or mobile? Is it just two stations or more?

[d00m178]

I already buy some from amazon:

https://www.amazon.com/gp/product/B01HHPK7KS/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

inside it looks like this:


wire with two springs.

I decided use GP antenna in past, and have several antennas already. So I think I will use them at least till the end of this season.

But anyway - for all the antennas I need feedline - RF cable with connectors and here I want to learn how to calculate the right length of this cable
As far I know - it is very important if I want build effective antenna system.

So would you please check my calculations in my previous post?

As for my project - I build my LoRa nodes for measuring different snow conditions in snow cover on mountains slopes.
This info used by avalanche services for forecasting avalaches.
It is non-commercial task - I just helping to friend of mine who work it this avalanche service check his theory about avalanches. 
Also it is very interesting for me to learn snow because I like freeride and backcountry  in mountains )
So because of all - it is kind of hobby.
Now we have 5 nodes (servers), all of them installed on different slopes and works from November till May.
Man with client device periodically go to safe points to get info from nodes. Usually two times during day.
I'm planning build kind of gateway for nodes that located in the same location and get all data from gateway.

So for these purposes I want build good antenna system.

[d00m178]

so...

any updates about my calculations? Im going to make a cable with connectors and would like to know what length to do.

[d00m178]

sorry but any updates?
I'm worry if my calculations is correct.

again:

I have cable with VF I decided to use is 1.28 so 1/2 wave length is 20.98 cm.

I need build feed line which contains: UFL connector soldered on PCB Moteino -> pigtail with SMA connector -> RF cable (SMA - N type connector) -> GP antenna based on N type connector.

The main question how to calculate part of length of RF cable?
If I understand all this info correctly (https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/connecting-sma-pigtail-to-moteino/msg15744/#msg15744) this length need to be calculated very accurate.
So total length of FEED LINE  from UFL to antenna (pigtail + RF cable + N type antenna ) should be multiple of 20.98cm due to "minimize RF going back to the transmitter"

I know length of pigtail - 26cm from end to end and length of N type connector on antenna side is 2 cm so total length of them is 28cm.

Next I need calculate length of RF cable. Let's say I have 400cm of cable. 

for example 20.98 multiple of 19 times == 398.62cm

then I need to cut 28cm for pigtail and N type connector lenght so finally I get 370.62cm of RF cable.

and total feed line will be RF cable (370.62cm) + pigtail and N type (28cm) == 398.62cm

Is it correct calculations?
Please take a look and advice.


and second question - can I use such feed line with GP antenna for 915Mhz with 1/4 wave length wires (82mm)?


Thank you!

[d00m178]

UPD:

after some investigations I noticed that vF 1.28 which I used in my previous calculations was wrong.
my cable has FPE dielectric so correct vF is 1/(1.28) == 0.78
so:
For coax with Vf = 0.78, 3/4 wavelength = 19.18 cm for freq 915MHz

19.18 instead 20.98 cm.