Hello Alexander
First I have to say I am not an expert on this topic. And nice choice of subject for a build.
Here is information I have, given in good faith. Any "experten" on the subject please correct if needed.
Kfz 17 was the designation for radio, telephone & survey vehicles, regardless of manufaturer: Horch, Steyr, etc (source Samochody Wehrmachtu Vol IV).
Numerous pictures exist of early model Kfz 17 (Horch) had the "bedframe" antenna, similar to what was used on other wehrmacht vehicles.
I have found one picture of a (Steyr) Kfz 17 with a bedframe antenna, (below) and others with no visible antenna. (last picture)

I surmise there are two other options - either a telescoping antenna, (sometimes attached to the rear of some radio vehicles) or a small antenna mount mounted on the side for a 1.4 metre antenna. Below is a Horch Kfz 17 with one simple aerial mount.

Given there are photos of Steyr 1500 Kfz 17 radio trucks with no
visible antenna you are probably safe to model without an antenna.

. The below picture is identified as a Kfz 17.
If you want some kind of antenna, suggest you make one of the 1.4 metre variety(from base of stretched sprue) or obtain a small mount from your spares box. I can spare a telescoping type antenna from a ZV Models Einheitswagen radio vehicle if you want it. (see below for telescoping antenna)

You could try scratchbuilding a bedframe antenna, as per the photo, I know you have the skills, though sadly the entire details of this antenna are not visible so impossible to replicate it exactly, so if you have one in the spares box from another vehicle, its another possibility.
Or if you make it a 'Telephone Car' it does not need an antenna, it had a strip on the rear for a cable to plug into to operate a telephone exchange for up to 20 lines (for use in a Command Post/Headquarters type of situation).
Sites for further information:
http://www.wehrmacht-awards.com/forums/showthread.php?t=625512http://www.kfzderwehrmacht.de/Homepage_english/Motor_Vehicles/Austria/Steyr/Steyr_270/steyr_270.htmledit - I started writing this post before the previous post by Robert appeared, seems we are more or less on the same track)
cheers
Neil