[aprssig] Cross Band Repeat on TM-D700A double as digi?

Stephen H. Smith wa8lmf2 at aol.com
Mon Mar 14 02:13:15 EDT 2011

On 3/13/2011 9:20 PM, Wayne Sanderson wrote:
> I am curious- If I set my mobile TM-D700A into Cross Band Repeat mode
> and set the A side to 144.39 and the B side to a packet segment of the
> 60cm band, might it bounce the APRS traffic onto 60cm and any 60cm
> APRS packets over to 2 meter? I live in a townhouse community where I
> can't have a home antenna, so my handhelds are my only home access to
> APRS and they don't hit the nearest digi from inside the house. If
> this works, I can keep a mini nub antenna on my D72 and run APRS on
> 60cm at low power and my mobile can be my gate into the 2 meter APRS
> network. Any thoughts?
I assume you mean 70 cm

The cross-band digipeating does work, but only one way since you define RX on 
one side and TX on the other.  In the mobile you will have to configure RX on B 
(UHF and TX on A (2M).    The hand-held's setup will be the opposite -- data RX 
on band A (two meters) and data TX on band B (UHF)     The result is that you 
will be transmitting with the aid of the mobile, but receiving direct on two 

I once used this to get out of an underground EOC to my car parked in the lot 
above.      I could hear one of the nearby digis direct, but couldn't get back 
to it with a TH-D7 indoors.   The crossband repeat in the D700 worked perfectly 
with TX on UHF and RX on VHF.    I have the internal digipeater in the D700 set 
to the recommended TEMP1-1 path, so I just added this hop to the hand-held's 
path and it worked perfectly!

Note that you have to access the D700 with a terminal program running on a 
computer (not an APRS application) to set the digipeater options that don't 
appear in the radio's setup menus, such as the digipeat alias of "TEMP1-1".   
I.e. you have to type command lines directly into the TNC by hand like with an 
old  TNC2 to set up the digi options.  (The complete set of direct TNC commands 
are in an appendix at the back of the D700 manual.)


Another "brute-force" approach is to use the cross-band repeat mode for voice.  
This CAN be set to digi in either direction -- whenever the squelch breaks on 
one band,whatever is heard is retransmitted on the other band.    No TNC is 
involved -- you are just retransmitting "packet racket" as through it was voice 
from one band to the other.  No added path is involved since no TNC processing 
of data is going on - you just leave your hand-held APRS setup the way you 
would use it on two meters.  The only difference is that you assign it's TNC to 
the UHF side of the radio (band B) instead of the VHF side (band A).

The real problems is this mode are that:

1)  Data isn't being regenerated by a TNC.  As long as the incoming data is 
full-quieting, it will go out OK, but if the path is marginal on the input 
side, you will be retransmitting a noisy signal on the output, just like a 
voice repeater with a user in a fringe area.

2)  If the 144.39 side has a lot of traffic, your mobile digi will be RXing 
2m-TXing UHF almost constantly. You may have trouble getting enough quiet time 
to transmit the other way.



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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