If a co-ax cable is not terminated in its characteristic impedance, you will get peaks and nulls in the voltage as you go along the length of the cable.
I have never heared of DS-3 before,
but I would expect that either:
In the equipment, there is a means to terminate the cable by switching in a suitable resistor.
or
You should use a T-piece and fit a terminator external to the equipment.
As I have been typing, another thought occurs to me, are you using the correct cable?
Co-ax is made with 50, 75 and sometimes 91 ohm characteristic impedance. Even when correctly terminated, using the wrong cable will give you some strange effects.
And as Nigel said, the effects will vary with the length of the cable.
Edit: I have just been Googling and found that DS3 uses 75ohm cable. If your cable is not 75ohm you could easily get problems.
JimB