Document:

<PAGE>   1
                                                                    EXHIBIT 10.2

                                  July 27, 2001

Morgenthaler Venture Partners V, L.P.

Gentlemen:

Reference is made to that certain Letter Agreement dated May 23, 2001, as
amended, (the "Agreement") among Morgenthaler Ventures Partners V, L.P.
("Morgenthaler") and Sheldahl, Inc. (the "Company").

This letter confirms our understanding that the following changes to the
Agreement shall be made effective as of July 22, 2001:

A.   The phrase "90 days from the Signing Date" set forth in the third sentence
     of Section 3 shall be replaced by "October 1, 2001."

B.   The phrase "during the first 60 days after the Signing Date" referenced in
     subparagraph (a) of the last sentence of Section 3 shall be replaced with
     "prior to October 1, 2001" and subparagraph (b) shall be deleted.

C.   The phrase "During the first 60 days following the Signing Date" referenced
     in the first paragraph of Section 4 shall be replaced with "Until October
     1, 2001...."

D.   The phrase "if during this 60-day period" referenced in the second
     paragraph of Section 4 shall be replaced with "if prior to October 1,
     2001...."

E.   The last sentence of the second paragraph of Section 4 shall exclude any
     obligation to reimburse a potential acquiror for its expenses in conducting
     due diligence with respect to the Materials Business in the event the
     business is not acquired by such potential acquiror, provided such expense
     reimbursement shall not exceed $50,000.

F.   The third paragraph of Section 4 of the Agreement shall be deleted.

G.   The phrase "the 90th day after the Signing Date" in the first sentence of
     Section 5 shall be replaced with "October 1, 2001."

H.   The $900,000 amount referenced in Section 5 shall be reduced to $500,000.

                                       1

<PAGE>   2

I.   The $300,000 amount referenced in Sections 4, 5 and 8 shall be $350,000,
     inclusive of amounts paid to Ampersand IV Limited Partnership per the
     letter among the Company, Morgenthaler and Ampersand dated this date.

J.   The phrase "within 90 days of the Signing Date" set forth in Section 8
     shall be replaced with "on or before October 1, 2001."

Please indicate your agreement to the terms hereof by signing this letter in the
space provided below.

                                        SHELDAHL, INC.

                                        By: /s/ Donald R. Friedman
                                            ------------------------------------
                                                Donald R. Friedman
                                                President and CEO

ACCEPTED AND AGREED:

                                         MORGENTHALER VENTURE PARTNERS V, L.P.

                                         By: /s/ John D. Lutsi
                                             -----------------------------------
                                                 John D. Lutsi
                                         Its:    General Partner

                                       2<PAGE>   1
                                                                     Exhibit 4.1

                             SUPPLEMENTAL INDENTURE
                                       OF
                                   GUARANTORS

         THIS SUPPLEMENTAL INDENTURE dated as of April 9, 2001 is delivered
pursuant to Section 10.04 of the Indenture dated as of November 15, 1996 (as
heretofore or hereafter modified and supplemented and in effect from time to
time, the "Indenture") among LAMAR MEDIA CORP., a Delaware corporation,
(formerly Lamar Advertising Company) certain of its subsidiaries ("Guarantors")
and STATE STREET BANK AND TRUST COMPANY, a Massachusetts banking corporation, as
Trustee ("Trustee") (all terms used herein without definition having the
meanings ascribed to them in the Indenture).

         The undersigned hereby agree that:

         1. The undersigned is a Guarantor under the Indenture with all of the
rights and obligations of a Guarantor thereunder.

         2. The undersigned hereby grants, ratifies and confirms the guarantee
provided for by Article Ten of the Indenture to guarantee unconditionally,
jointly and severally with the other Guarantors, to each Holder of a Note
authenticated and delivered by the Trustee, and to the Trustee on behalf of such
Holder, the due and punctual payment of the principal of (and premium, if any)
and interest on such Note when and as the same shall become due and payable.

         3. The undersigned hereby represents and warrants that the
representations and warranties set forth in the Indenture, to the extent
relating to the undersigned as Guarantor, are correct on and as of the date
hereof.

         4. All notices, requests and other communications provided for in the
Indenture should be delivered to the undersigned at the address specified in
Section 12.02 of the Indenture.

         5. A counterpart of this Supplemental Indenture may be attached to any
counterpart of the Indenture.

         6. This Supplemental Indenture shall be governed by and construed in
accordance with the laws of the State of New York.

<PAGE>   2

         IN WITNESS WHEREOF, the undersigned have caused this Supplemental
Indenture to be duly executed as of the day and year first above written.

                                        Guarantor:

                                        Lamar Bellows Outdoor Advertising, Inc.

                                        By:  /s/ Keith A. Istre
                                             ----------------------------------
                                             Keith A. Istre
                                             Vice President - Finance and Chief
                                             Financial Officer
Attest:

By:      /s/ James R. McIlwain
   -------------------------------------------
     James R. McIlwain, Secretary

Accepted:

STATE STREET BANK AND TRUST
         COMPANY, as Trustee

By:      /s/ Andrew L. Sciorz
   -------------------------------------------

Title:   Assistant Vice President
      ----------------------------------------

                                       2
<PAGE>   3

                        ADDITIONAL SUBSIDIARY GUARANTORS

McCloskey Outdoor Advertising, Inc.

Lamar Advertising Southwest, Inc.

Lamar DOA Tennesee, Inc.

Lamar DOA Tennesee Holdings, Inc.<PAGE>   1
                                                                     Exhibit 4.2

                             SUPPLEMENTAL INDENTURE
                       TO INDENTURE DATED AUGUST 15, 1997

         THIS SUPPLEMENTAL INDENTURE dated as of April 9, 2001, is delivered
pursuant to Section 4.11 of the Indenture dated as of August 15, 1997 (as
heretofore or hereafter modified and supplemented and in effect from time to
time, the "1997 Indenture") among OUTDOOR COMMUNICATIONS, INC., a Delaware
corporation, certain of its subsidiaries (the "Guarantors") and FIRST UNION
NATIONAL BANK, a national banking corporation, as Trustee (the "Trustee") (all
terms used herein without definition having the meanings ascribed to them in the
1997 Indenture).

         The undersigned hereby agrees that:

         1. The undersigned is a Guarantor under the 1997 Indenture with all of
the rights and obligations of the Guarantors thereunder.

         2. The undersigned has granted, ratified and confirmed, in the form and
substance of Exhibit B to the 1997 Indenture, the Guarantee provided for by
Article XI of the 1997 Indenture.

         3. The undersigned hereby represents and warrants that the
representations and warranties set forth in the 1997 Indenture, to the extent
relating to the undersigned as Guarantor, are correct on and as of the date
hereof.

         4. All notices, requests and other communications provided for in the
1997 Indenture should be delivered to the undersigned at the following address:

                  Keith A. Istre
                  Vice President - Finance and
                  Chief Financial Officer
                  Lamar Media Corp. and its Subsidiaries
                  5551 Corporate Blvd.
                  Baton Rouge, LA 70808

         5. A counterpart of this Supplemental Indenture may be attached to any
counterpart of the 1997 Indenture.

         6. This Supplemental Indenture shall be governed by and construed in
accordance with the internal laws of the State of New York.

<PAGE>   2

         IN WITNESS WHEREOF, the undersigned have caused this Supplemental
Indenture to be duly executed as of the day and year first above written.

                                   Guarantor:

                                   Lamar Bellows Outdoor Advertising, Inc.,
                                   a Nebraska Corporation

                                   By:  /s/ Keith A. Istre
                                        -----------------------------------
                                        Keith A. Istre
                                        Vice President - Finance and Chief
                                        Financial Officer

Attest:

By:      /s/ James R. McIlwain
   -----------------------------------------
     James R. McIlwain, Secretary

Accepted:

FIRST UNION NATIONAL BANK, as Trustee

By:  /s/ James Long
   -----------------------------------------
Title:   Assistant Vice President
      --------------------------------------

                                       2

<PAGE>   3

                        ADDITIONAL SUBSIDIARY GUARANTORS

McCloskey Outdoor Advertising, Inc.

Lamar Advertising Southwest, Inc.

Lamar DOA Tennesee, Inc.

Lamar DOA Tennesee Holdings, Inc.<PAGE>   1
                                                                     Exhibit 4.3

                             SUPPLEMENTAL INDENTURE
                                       OF
                                    GUARANTOR

         THIS SUPPLEMENTAL INDENTURE dated as of April 9, 2001, is delivered
pursuant to Section 10.04 of the Indenture dated as of September 25, 1997 (as
heretofore or hereafter modified and supplemented and in effect from time to
time, the "Indenture") among LAMAR MEDIA CORP., a Delaware corporation, certain
of its subsidiaries ("Guarantors") and STATE STREET BANK AND TRUST COMPANY, a
Massachusetts banking corporation, as Trustee ("Trustee") (all terms used herein
without definition having the meanings ascribed to them in the Indenture).

         The undersigned hereby agree that:

         1. The undersigned is a Guarantor under the Indenture with all of the
rights and obligations of Guarantors thereunder.

         2. The undersigned hereby grants, ratifies and confirms the guarantee
provided for by Article Ten of the Indenture to guarantee unconditionally,
jointly and severally with the other Guarantors, to each Holder of a Note
authenticated and delivered by the Trustee, and to the Trustee on behalf of such
Holder, the due and punctual payment of the principal of (and premium, if any)
and interest on such Note when and as the same shall become due and payable.

         3. The undersigned hereby represents and warrants that the
representations and warranties set forth in the Indenture, to the extent
relating to the undersigned as Guarantor, are correct on and as of the date
hereof.

         4. All notices, requests and other communications provided for in the
Indenture should be delivered to the undersigned at the address specified in
Section 12.02 of the Indenture.

         5. A counterpart of this Supplemental Indenture may be attached to any
counterpart of the Indenture.

         6. This Supplemental Indenture shall be governed by and construed in
accordance with the laws of the State of New York.

<PAGE>   2

         IN WITNESS WHEREOF, the undersigned has caused this Supplemental
Indenture to be duly executed as of the day and year first above written.

                                   Guarantor:

                                   Lamar Bellows Outdoor Advertising, Inc.

                                   By:  /s/ Keith A. Istre
                                        ----------------------------------------
                                         Keith A. Istre
                                         Vice President - Finance and Chief
                                         Financial Officer
Attest:

By:   /s/ James R. McIlwain
   -----------------------------------------
     James R. McIlwain, Secretary

Accepted:

STATE STREET BANK AND TRUST
    COMPANY, as Trustee

By:      /s/ Andrew L. Sciorz
   -----------------------------------------
Title:   Assistant Vice President
      --------------------------------------

                                       2
<PAGE>   3

                        ADDITIONAL SUBSIDIARY GUARANTORS

McCloskey Outdoor Advertising, Inc.

Lamar Advertising Southwest, Inc.

Lamar DOA Tennesee, Inc.

Lamar DOA Tennesee Holdings, Inc.

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