Relation between FNMA-30 coupon & Loan Interest Rate

I am a newbie here, can any one explain the relationship between FNMA-30  coupon  & Loan Interest Rate? Let's say, assume that today FNMA-30 4.5% coupon 101.13, anyway we can get the loan interest rate range?  any formula or rule of thumb for this relation? Thanks in advance

Welcome Charl...to help you understand some of this information you can go under the blogs/news tab above and go the the MBS Commentary on the right handside you will see MBS basics.  This will give you some basic information.

There is not a symbol that you can look up. 

In regard to the impact of mortgage loan interest rate change between FNMA-30  coupon  and 30 Year Treasury Bond or 10 year treasury note, which one has big impact?

My vote would have to be for the FNMA 30 coupon of those 3 choices.  The GNMA's behave similarly to the FNMA as well.  The "old school" method for most LO's, was to watch the 10 yr Treasy, but in this global economy that we are in, there are many pressures unrelated to mortgage lending that move the 10 yr in various directions.  The MBS blog is an invaluable tool that can help you time the market on long and short term float/lock decisions.  If you are a consumer, I'd recommend working with a broker/LO that understands MBS - they will be able to give you the best advice on your float/lock decision.

the general rule that i use is each point represents a .25%, and lenders want to make about 1 ysp,  so if the 4.5 is at 101-13, then par on our rate sheet should be 4.375.  If the coupon was at 102 then we should see 4.25.  Once the coupon gets over 102, then you really need to watch the lower coupon.