According to the Google URL Builder document, utm_source, utm_medium and utm_campain are mandatory.

What happens in Google Analytics when you only fill out the latter two, for example:

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George answered yours question best.

But why do you want to leave utm_source blank? Looking at yours example link I think that you would like to dynamically fill it with referrer.

Lunametric's posted about that: http://www.lunametrics.com/blog/2014/06/11/campaign-tracking-dynamic-source/

According to that article you can do it in a two different ways: using GTM or modifying a GA code. Here I paste the second one.

1. tag your links


2. add js code before your GA tracking code

js code_pict

3. modify a GA code


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You can find the information you want by looking at the following chart. It shows how each session's campaign & traffic source is processed in the servers of Google.

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You can easily try it out and check in real time reports under traffic sources to see what happens. Likely you would get (not set) as the source is required to properly set your cookie.

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utm_source is definitely required for the other parameters to be recorded.

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