ROI from advertising?

Any suggestions for how to track how many site visitors found a web
site from a print or radio ad?

Some background about the nature of our site:
It is not an e-commerce site. Our company offers customized training
and engineering services. While we do have some products that we sell,
we do not sell them directly from our site, but provide them as part
of customized solutions. The purpose of the site is primarily to
inform people about our capabilities, and to encourage them to contact
us so that we can start establishing a relationship with them.

For a while, we had a popup survey that asked how people heard about
our site. We averaged 22 responses a day  (our current average visitor
sessions per day = 264, and we only popped up the survey once per
cookie-enabled visitor, whether they answered or not). Then we got
complaints about the "unprofessional" nature of the popup survey -
from within our company, not from site visitors - so we switched to an
opt-in survey.

Our response rate dropped drastically - fewer than 20 in a month. This
low response rate persisted even when we offered a drawing ($50 gift
certificate to giftcertificates.com) and animated (single-loop) the
survey button.

We would REALLY like to know how many site visitors come to us as a
result of seeing a print ad.

We have thought about:

1. Using a unique URL in the print ads (eg, www.ourcompany.com/info/),
and having that page simply redirect to the home page - but as part of
the redirection, we could log an "ad hit".

2. Using a modified domain name for the ad URLs (eg,
info.ourcompany.com/).

3.  Using a specific URL for each type of ad (eg,
www.ourcompany.com/powerplants/, www.ourcompany.com/reliability/), and
having an ad-related page with links to information we think that
people responding to that ad might find useful.

Any suggestions will be appreciated.

Thanks!

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