Um....
This post is all over the place and perhaps even off-topic. Maybe not,
but regardless, it's an interesting topic to me so I'll try to address
some points from my perspective.
Please see inline comments below...
On 8/11/2017 11:29 AM, Josh Branning wrote:
Hi,
Recently I attempted a 9km communication via 4W CB radios and a
homemade 1/2 wave dipole, from a rural village, to a town. I figured
with more power and a low frequency, they might fare better for
unlicensed communication. (I am based in the UK.) Unfortunately, I
didn't manage a voice signal over that distance.
Yeah, even with a 1/2 wave dipole that's a bit of a reach. How high did
you mount the dipole? Was it constructed to G5RV specs or specifically
to the 11 meters (CB) band? G5RVs usually work great with a little
tuning for 11 meters as well.
Here's some info on the awesomeness that is a G5RV:
http://www.hamuniverse.com/g5rv.html
CB communication over AM and LSB is usually conducted with vertical
polarization, and for such, if you're using a dipole I don't think it's
really practical to locate a place that high to hang a dipole, but an
inverted V configuration will usually suffice the needs for both
horizontal and vertical polarization applications.
CB/11 meters AM communication maxes out w/5 Watts at about five miles,
line of sight, under ideal conditions. Sun spot activity will have an
adverse affect on that but conversely, you should be picking up and be
able to communicate with folks on the other side of the globe due to the
enhancements that the most active periods of the 11 year sunspot cycle
affords skip.
I'm here in California, so the laws are a little different, but
probably not much, as the FCC lays down the rules for licensed and
unlicensed bands. You're talking about HAM modes below over CB band,
which I'm not sure is actually legal anyway, but you would know better
what the regulations are for the unlicensed 11 meters spectrum - Over
here I believe that voice only, over AM and SSB are permitted on
Citizens Band.
As far as satellite communications is concerned, you generally use
circular polarization to bounce off of OSCARs and such, and yet you're
talking about digital PSK and I wasn't aware that anything up there is
listening on those frequencies. Here in the states, FM is not permitted
over CB (If I recall correctly).
Someone other, who is into amateur radio told me it may be to do with
being at the peak of a solar cycle. I have only made contact via CB
within the village, and major road where truckers pass. I have tried
(thus far unsuccessfully) to make contacts using digital modes such as
BPSK31 and JT-65 over FM. I use a CB radio with built in VOX, as it is
easier than building the VOX circuitry or PPT switch. I am yet to set
up an realtek SDR dongle at the other location so I can try *really*
slow PSK to see if I can make a digital link by myself (in voice
tests, the other radio did not have VOX). [1]
I am also looking into LoRa transceivers, which claim up to 15km
distance. Unfortunately, most actual/web reports only give a 2-5km range.
I visited the local amateur radio club, and they seem to think a
extremely efficient and small narrow band signal will tend to travel
further, though personally I have doubts if that advantage can be
produced in software (eg. the transmitter has to probably be designed
for the narrow bandwidth).
Yes, a Yagi or other multi-element beam of some sort would provide you
added coverage over a more narrow area where line of sight is concerned,
but the dipole is likely to be a top performer with skip.
I have found a potential method of communicating from/to satellites,
but (?legally?) you'd probably need to ask someone for permission to
use the network [2] ... tehehe.
Like I said, I don't think that any satellites are listening anyway, for
any digital modes of communications in that part of the HF spectrum
(CB/11M). There is *some* 6 Meter stuff, but most is 2 Meters and up -
for that, use a beam and check your charts/tables for when they will be
overhead.
The good:
1) satellites
2) *many* ground nodes
3) AX.25 unnumbered frames, meaning truly distributed routing, as
opposed to IP.
4) runs on different modulation schemes
5) has bidirectional internet gateways
6) anyone can run a gateway/network
7) can send text messages and other (text messages tested, other not)
8) can run on microcontrollers
9) free as in freedom
10) only cost of setup, else, free as in beer (no connection fee)
11) email servers/gateways exist
12) IP can run atop of AX.25
The bad:
1) Generally uses amateur radio rather than unlicenced frequencies.
(though could use other.)
This is correct. The first part anyway - I don't know what you mean by
'other'.
2) Generally uses FM. (though could use other.)
SSB too over VHF, some UHF.
3) Possibility of getting a hobbyist into trouble and worse, getting
complete lock-down/regulation.
I don't know what that means, but yes, you might get into trouble, even
though it is unlicensed spectrum, because the mode isn't permitted (and
again, who's listening?)
4) Not generally spread spectrum.
5) easy-to-spoof delivery mechanism
Okay for that last point there, (#5), I see no relevance there - Over
HAM bands, if you're licensed, you are prohibited from ANY sort of
business communication (that includes telling your wife to order a pizza
for when you get home). So what possible issues would you be concerned
about with people 'spoofing'?
Unknown:
1) voice/images
2) cellular phone network integration
That said, I was also looking at the frequency/distance relationship.
Problem is the lower the frequency, the bigger the antenna you need.
Obviously you can transmit with a small antenna if the impedance is
close (using chokes etc.) but that requires *major* custom hardware,
which is somewhat impractical.
Um... you can work with a half-wave or quarter-wave radiator, and even a
J-Pole design for VHF works surprisingly well if all you have is attic
space or some other limiting space factor.
As far as "major hardware", I would offer that such is not the case.
Hams are all about DIY and tinkering, and if you look around you'll see
design and kit ideas and plans that you can scrape together from
inexpensive hardware at your local Home Depot (or whatever hardware
store you have in your neck of the woods) for a pittance. Yes, you could
spend some bucks but why, when you can achieve the same performance with
commodity hardware?
One thing you've omitted, and this is with respect to your dipole -
what are you doing about the tuning of your antennae with respect to
your standing wave ratio? If you've attended some local Ham groups
there, you've probably noticed that folks are really eager to help out
with a challenge and the opportunity to help someone else. See if
someone there has like a Telewave or Bird Watt Meter or something along
those lines. They make them for VHF/UHF and also HF. Any Watt meter for
your band will do, and if you can't borrow one of those then an SWR
meter is second best.
Put the Watt meter in series with your antennae feed line, key the mike
when you have selected to measure the forward power, note the reading,
then release the PTT. Then switch the meter to measure the reflected
power and key the mike again, noting how many Watts are coming back -
too much can damage your transceiver, although in your case we're
talking about a CB with no more than 4W of output.
Take the first measurement and divide it by the second and this will
yield an accurate standing wave ratio for you. i.e.,
4 Watts forward vs. 0 Watts reflected is ideal (although not attainable,
there is always some reflected power), but if you're putting out 3.7 W
and 2.2 W reflected power = roughtly an SWR of well over 7 - this could
damage your transceiver, perhaps even at such a low wattage, as the
radio simply isn't designed to have that much power floating back at it.
If you're putting out 3.7W and 200mw of reflected power, that's an SWR
of about 1.6 which is pretty good. Not awesome, but anything under 1.5
is just an exercise in perfection anyway and there won't be much
realized improvement from there.
I did a quick Bing and came up with this for you:
How to use a Watt meter: https://www.youtube.com/watch?v=fbj_r6QfN4I
Here's an online SWR calculator to make things simple for you:
http://rfcalculator.mobi/vswr-forward-reverse-power.html
Now, although this will vary from transceiver to transceiver, I'll just
quickly talk about tuning your 1/2 Wave dipole. After checking your SWR
if the Antennae needs to be shortened a smidgen, then you can snip off
equal lengths from both sides of the dipole (each radiating element
equally!).
If it needs to be a bit longer (and this also works if it needs to be
shorter), an external antennae tuner can most often bring your SWR down
to, or at least close to 1.5 SWR (how? some of your power is absorbed
but the better antennae match more than makes up for this where your
propagated signal is concerned!). MFJ makes some great and reasonably
priced tuners - and do check on eBay for the very best deals ;).
Well I hope that helps and serves to set you on a course of resolution -
in a nutshell, even though at line of sight with a CB, if you were using
your dipole hung either either vertically (from a tall tree or
something) or in an inverted V configuration, I would think that you
would have a bit more reach than the standard 1/4 wave magnetic mount
mobile antennas could afford you. Perhaps you might consider something
like a cushcraft R8 or some other vertical like that with some grounding
planes and an antennae tuner (Now we're talking about an outlay of cash
on hardware lol).
One more thing..... Please consider passing your tech exam. There's a
whole world waiting for you with just the sort of problems, solutions,
and satisfaction that you brought here to the discussion list that I
believe you would revel in, within the confines of the band plan for
licensed amateur radio operators. I'm sure your local club has loaner
exam books and will be more than happy to mentor you through passing
your exam too - then you can haz #cheezburgerz!
Kindest regards,
--
Bradley D. Thornton
Manager Network Services
http://NorthTech.US
TEL: +1.310.421.8268
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