|
VoIP Article for January
2005 SERA Journal
Choosing a Link Radio for Internet Linked Repeaters
Danny Musten KD4RAA
One
of the frequent questions that I am asked is "what link radio is my choice for
interfacing IRLP or Echolink to repeaters"?
I have experimented with many and found some more suitable for linking
than others. For certain, the stock amateur mobile rig is not one of them.
Although ham gear is frequency agile, it often lacks the necessary logic and
convenient audio access points that are needed to interface to the VoIP system.
Selectivity is usually poor in high RF environments and most importantly,
does not normally offer continuous duty operation in the power amp stages. Link
radios can stay keyed on and off for hours on some of the common regional nets
and get mighty warm with those extended QSO's on the VoIP system.
You also might have noticed that the PL encoder/decoders in some ham rigs
have a slower response. Save your ham rig to make the personal node link in the
shack, and dedicate an older proven GE or Motorola product to the repeater link
application.
If you are familiar with building repeaters
or helping maintain them, you will not be afraid to tackle interfacing these
links. For the faint of heart, there are some plug and play solutions out there
for some of the common ham rigs, but performance will vary and I would not
encourage them for a long-term solution, especially if left linked to your local
repeater. I recently saw a back to
back post on one of the user groups from one fellow that had successfully
"hooked up" his HT-202 simplex to his node, boasting how his "vox
worked real good" and that it was "running off of his computer's power
supply to a 50 foot Ringo". Well, can you name at least five things wrong
with this picture? I couldn't help
but chuckle after noting that his follow-up was posted a few weeks later asking
for recommendations for the "best link radio to use".
Some of the packet ready radios are real tempting because they have those
data ports for logic and audio access, but suffer on the TX side when it comes
to continuous duty. Composing this article, I am reading a node user's note on
the IRLP owners group indicating that his Yaesu mobile just gave up the ghost
due to finals failure.
If you do decide to homebrew your own interface (my schematics were in the
August 2004 article) and experiment with that extra rig you have lying around to
build up your personal node, use common sense and good amateur practice -
operate on a vacant 70cm or 1.25 meter simplex with the lowest power setting and
even then, put a fan on the finals. Simplex nodes are a whole 'nother topic,
especially full-power 2-mtr ones without PL protection.
Just be sure that you if make your simplex node available on your tall
tower, that you are able to simultaneously hear the locals out there using your
node or you will leave yourself open for interference.
I have to tell you that I have received at least two inquiries on how to
interface an IC-706 and one FT-847 to their local repeater… that, without
hesitation I offered to "trade" them for a commercial rig ready to
roll. Why would anyone waste… well, if you are one of these, I will be more
than happy to talk to you privately!
I really want to encourage you to have the
right configuration in place before you make the VoIP network available on your
local machine. Good used commercial mobile radios are easy
to find these days at hamfests, ebay, state surplus sales, etc…with many of
mine rescued from the dusty shelves of local two-way shops or some hand-me-downs
from commercial agencies that have been otherwise sold on utilizing other bands
or the latest bells and whistles technology. Most commercial gear is built with
better front-end selectivity and rejection, have documented places to pick off
your logic and audio connections and generally are rated better on the RF decks
for industrial duty. I will mention
some of my favorites in a moment, but first, there are some considerations to
make when connecting to one the VoIP networks.
Your repeater is the first thing you want to make sure is ready before
you install the link.
If you run a 220, UHF or higher frequency machine, then you can link directly to
the repeater with your VoIP link radio. If
you have a 2M or 6M machine, then according to present rules, you need a
coordinated link hanging off your repeater or from one of your controller ports
and of course, a matching rig on the VoIP box end.
If you are fortunate enough to have internet at your transmitter site,
then you can direct interface your VoIP box to an unused controller port and
eliminate the RF link.
To keep your link to the VoIP network interference free, it is extremely
important and even required on some of the Reflectors and Conferences, for you
to eliminate the audio from your repeater's courtesy tones, announcements and
ID's. Hang time is also frowned
upon as those on the "other end" have to wait for your repeater to
reset before anyone else can talk. Anyone
who is involved in linking repeaters knows the importance of not having YOUR
information come across on other repeaters or links, especially when in large
nets on the VoIP network, that can have literally dozens of machines connected
in. If all repeaters linked in sent their controller's personality, we would
have one big mess. Some linked repeaters are configured for instant drop -
allowing for fast response and recovery times on the links, even when they use
CTCSS. Your hang times and courtesy tones would not only slow the net down, by
us having to wait for your system to finish, you instantly become an unwanted
celebrity and may even get blocked or muted from further participation.
Besides, the repeater owners who have worked hard on their system to
configure it properly get extremely annoyed when others do not conform to this
"clean audio" standard. The solution is really very easy by either: 1,
placing your controller/repeater on demand in zero hang time or 2, use PL to
gate your audio to the network.
If you already have PL decode
and re-encode on your repeater, then you are all but home free. Many repeater
owners have already configured their machines to re-encode the PL on the TX side
- ONLY when there is a valid PL or carrier on the receive side of the repeater.
All of the announcements, courtesy tones, ID's…etc. are sent without
the PL on the transmit, thus your link radio, that is hanging off of your VoIP
box, will hear voice audio only. Those
who frequently cross-band repeat from their HT's to their mobiles know that the
response/recovery time is far better when PL is re-encoded on the repeater.
With your VoIP link programmed to decode PL, you are set, but make sure
that your COS logic used to trigger the VoIP box is able to "see" the
PL decoder logic as opposed to just the COS sense only…or your link will be
hearing all that the repeater dishes out. Note that it is not necessary to have
PL decode on your repeater's input as long as the transmit side sends PL,
triggered from the receiver side COS. Your controller can obviously decide
whether of not to use PL decode, that really doesn’t matter except for the
benefit of your local users. But note that if your repeater is prone to intermod
or other interference, it can possibly sneak through to the network if you
happen to leave your repeater in carrier access connected and unattended. You
obviously risk being blocked from participating on Conferences and Reflectors.
One of the easiest ways to
implement this TX-PL encode - follow your receiver COS and/or PL decode is to
retro-fit your repeater using a relatively inexpensive Com-Spec TS-64 CTCSS
encoder/decoder. The encoder side
of this tone board can be made to trigger off of the decoder or repeater
receiver COS, as opposed to what you might find from stock commercial repeaters
that have PL encode full time while the transmitter is up.
All of your announcements, ID's, and even hang time can still be present,
but because they are sent without the PL tone, your link radio on the VoIP box
will never hear them. A quick look at the schematic and hookup instructions
available on the com-spec.com site, shows that you can tie the board's Transmit
PTT Input (that starts the encoder out) to your COS logic and it will encode out
- only on a valid receiver COS in. This
input pulls low to turn on the encoder, so you might need to add an NPN open
collector switch if your repeater receiver COS logic goes Hi.
Alternately, you could also tie the Transmit PTT Input to
the Audio Mute line out, which can be configured with a board jumper to pull low
when PL is decoded by the tone board.
Many modern controllers have
this TX PL encode out follow RX COS feature set built into the programming and
you may use your existing repeater tone boards to run a similar configuration.
Also make sure you don't have two PL encoders running at the same time, use a
high pass filter (strip the PL) on your receive audio and be sure to insert the
PL encode audio back in to your exciter at the right spot (not the same as the
voice audio path) or you will have some interesting "tonal" artifacts.
Another trick that I have
seen implemented, is that when the VoIP box/link makes an initial connection to
the repeater, it sends a custom set of DTMF tones that places the repeater
controller in a near zero hang time state, changes out the normal announcement
and courtesy tone macros, and has a "patient" ID that waits for a
quiet segment to run. When the node disconnects, it sends the DTMF codes to
place the repeater back to a normal state. Many repeater owners use this or a
similar feature set to alert them when the link is up. Plus it accomplishes
basically the same task as using the PL scheme, but on occasions, a forced ID or
burst of intermod will sneak through to the network if it is not behind PL.
Alternately, if you allow
your local users DTMF access, a custom macro can also be sent to the controller
by the operator to place it in a VoIP ready state, with some of the settings
mentioned above, including passing DTMF tones through to the node link and
turning on the PL.
On my local two-meter
machine, there is a dedicated UHF link on the 2nd port of the controller that is
dedicated to listening to the IRLP link radio/internet connection just a few
miles away. It runs less that 10 watts on both ends and uses yagis at 20
feet for the short hop. An old ACC-850 is still hangin' in there, but as many
know, requires a " # " prefix to allow DTMF to pass. The controller is
set to have one short beep when a local user is on the receiver side of the
repeater and a different chord tone heard on the repeater's output when the COS
is keyed from the link radio (VoIP) side. In
this way I can tell if the internet link is up and where the traffic is
originating. All of the audio is
configured behind the CTCSS tones and the participants on the VoIP side only
hear voice audio. The IRLP box also
runs a custom script that ID's the UHF link radio back to the repeater.
When choosing a link radio,
you will want to find one that has good access to:
1- Carrier Squelch Logic
(COS, RUS, etc.) follow your CTCSS or Digital Squelch Tone Decoder out, which
commonly swings +5 volts when your receiver is unsquelched AND decoding PL.
(This voltage swing indicator is headed to the IRLP interface board COS
input or Echolink interface to start the audio stream to the internet)
2- Line Level Receive Audio that is
de-emphasized, squelch-gated and PL filtered - hopefully not the speaker audio.
(this is headed for your Soundcard line input)
3- PTT
(pull low / open collector key from IRLP board or Echolink serial
interface adaptor)
4- TX audio inputs,
pre-emphasized and deviation limited. Mic audio will work here too. (The audio
source is coming from the line output of the soundcard.)
5.
Adjust the RF power out.
The two radios that I have
found to be most frequently used are the GE Phoenix SX and the Motorola
GM300/Maxtrac/Radius M-Series with the 16-pin rear accessory jack.
Some of the purists have converted their rock bound Mastr II mobiles,
Exec's, Micors and Mitreks, which seem well suited for the highest RF
environments while maintaining that Direct FM audio throughout. GE Rangers and
Deltas have also proven to be good choices, but like the MII's, Mitreks and
Syntors, they need a little work if you want to eliminate the long cable harness
and control head. I have also had some success with the GE MLS radios, but note
that the MLS I radios will not reliably hold the TX VCO below 446 MHz while the
MLS II's seem to behave better. The Phoenix as well as the 16-pin Maxtrac and
GM300s have all of the necessary audio and logic ins and outs available on the
rear accessory connector without having to hack into the rig.
There is a jumper in the M-Series vintage radios that allow flat or de-emph
audio to the 16-pin accessory jack. If
you do happen to have some of the 5-pin Maxtrac or Radius mobiles then I can
email you a hack to get COS and audio to the RJ45 mic jack in the front, which
is also documented on the batlabs and repeater-builders site.
Note that you will need access to software and/or a RIB box to program
the synth rigs, which many of us already have.
From experience, you may need
to add a DC blocking cap (10uf or so) and an audio pad (2k-50k) into your TX
audio path. There is usually some voltage on the mic line to power stuff in the
hand mic, if left present, might change your impedance or give other problems.
And since you are using line level out of the sound card, you might pad that
down some so that you will not overdrive the mic pre-amp stage in your radio.
A diode and maybe a
current-limiting resistor can be inserted in your interface harness to protect
your radio's COS logic from loading down. If you use the unprocessed receive
audio, a resistor/cap RC network can be used to flatten out or de-emphasize your
audio that points to your line-in of the soundcard. You will also want to have a
common ground between your supply and computer to reduce ground loop hum on the
audio path and provide a ground path for the PTT and COS logic. If you do have a grounding hum issue, then 1:1 isolation
transformers can be used in the audio paths, but make sure you don't forget to
provide a ground for the COS and PTT logic to work. Ah, and don't forget to
program or jumper the "hook" or "hang-up" feature for the
tone decoder to work. Check on your time-out timer programming as well.
Also, it is handy to program a simplex frequency with a PL tone into your
link to aid in setup and any audio fine-tuning before you place it on the
repeater pair. Finally, put a fan on your fins, have a properly rated DC
supply in place and have a go on the VoIP DVR test channels.
A
good place to find radio-specific information is on the repeater-builder.com
site. It has a wealth of links to GE, Motorola and various other gear.
If you have had good success with any other
rig as links, I would like to hear from you.
Danny kd4raa@sera.org . copyright
January 2005
|