<PAGE>

                                                                    EXHIBIT 4.12

THE SECURITIES REPRESENTED BY THIS NOTE AND THE SECURITIES ISSUABLE UPON ITS
CONVERSION HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED
(THE "ACT"), OR SECURITIES LAWS OF ANY STATE AND MAY NOT BE OFFERED, SOLD,
ASSIGNED, PLEDGED TRANSFERRED OR OTHERWISE DISPOSED OF IN THE ABSENCE OF AN
EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT AND APPLICABLE STATE SECURITIES
LAWS OR PURSUANT TO AN AVAILABLE EXEMPTION FROM REGISTRATION UNDER THE ACT OR
SUCH LAWS AND, IF REQUESTED BY THE COMPANY, UPON DELIVERY OF AN OPINION OF
COUNSEL REASONABLY SATISFACTORY TO THE COMPANY THAT THE PROPOSED TRANSFER IS
EXEMPT FROM THE ACT OR SUCH LAWS.

                       SENIOR CONVERTIBLE PROMISSORY NOTE

$                                                                         , 2002

     FOR VALUE RECEIVED, Proxim Corporation, a Delaware corporation (the
"Maker"), hereby unconditionally promises to pay to the order of           (the
"Holder" or the "Lender"), having an address at           , at such address or
at such other place as may be designated in writing by the Holder, or its
permitted assigns, the original aggregate principal sum of
($          .00), together with interest on the unpaid principal balance of this
Note outstanding at a rate per annum equal to ten percent (10%) (computed on the
basis of the actual number of days elapsed in a 365-day year) compounded
semi-annually and shall continue on the outstanding principal until this Note is
converted or paid in full. All payments of principal and interest by the Maker
under this Note shall be made in cash in immediately available funds.

     From and after the Maturity Date, every amount due and owing under this
Note shall bear interest at an annual rate of twelve percent (12%) (the "Default
Interest Rate"). Commencing after the Maturity Date, any accrued but unpaid
interest on this Note shall be payable on demand and shall accrue until the
obligation of the Maker with respect to the payment of such interest has been
discharged (whether before or after judgment). In no event shall any interest
charged, collected or reserved under this Note exceed the maximum rate then
permitted by applicable law and if any such payment is paid by the Maker, then
such excess sum shall be credited by the Holder as a payment of principal.

     1.  Definitions.  Unless the context otherwise requires, when used herein
the following terms shall have the meaning indicated:

     "Certificate of Designation" shall mean the Maker's Certificate of
Designations, Preferences and Rights of the Preferred Stock.

     "Change of Control" shall have the meaning ascribed to such term in the
Certificate of Designation.

     "Conversion Date" shall mean the date on which the stockholders of the
Maker approve the Proposal, whether at the Special Meeting or otherwise.

     "Maturity Date" shall mean (i) the date on which any Event of Default shall
have occurred and be continuing or (ii) the earlier of (a) any date, occurring
at least 120 days after the Negative Vote Date, on which the Holder shall have
demanded payment of this Note and (b) December 31, 2002.

     "Negative Vote Date" shall mean the date on which the Maker's stockholders
reject or fail to approve for any reason the Proposal at the Special Meeting.

     "Preferred Stock" shall mean the shares of Series A Convertible Preferred
Stock, par value $.01 per share, of the Maker.

     "Proposal" shall mean the proposal to be submitted to the stockholders of
the Maker at the Special Meeting to approve the issuance of the Preferred Stock
(as defined in the Purchase Agreement) upon conversion of the Notes and the
issuance of the Warrants to be issued at the Second Closing (as defined in the
Purchase Agreement) in accordance with the terms of the Purchase Agreement.

                                        1
<PAGE>

     "Purchase Agreement" shall mean the Securities Purchase Agreement, dated as
of June 16, 2002, between the Maker and the other parties named therein.

     "Special Meeting" shall mean the special meeting of stockholders of the
Maker to be called by the Maker for the sole purpose of approving the Proposal.

     2.  Securities Purchase Agreement.  This Senior Convertible Promissory Note
(this "Note") is one of the several promissory notes of the Maker (the "Notes")
referred to in the Purchase Agreement. The Note is subject to the terms and
conditions of the Purchase Agreement. The Note is transferable and assignable to
any person to whom such transfer is permissible under applicable law. The Maker
agrees to issue from time to time replacement Notes in the form hereof to
facilitate such transfers and assignments. In addition, after delivery of an
indemnity in form and substance satisfactory to the Maker, the Maker also agrees
to issue a replacement Note if the Note is lost, stolen, mutilated or destroyed.

     3.  Conversion.

     (a) On the Conversion Date, all outstanding principal on this Note shall
immediately and automatically be converted into such number of shares of
Preferred Stock as shall be equal to the quotient obtained by dividing the
aggregate outstanding principal balance then outstanding on this Note on the
Conversion Date by $25.00. In the event this Note is so converted, the Maker
shall pay the Holder on the Conversion Date an amount in cash equal to 20% of
the accrued but unpaid interest as of such date and the Holder shall forfeit any
remaining accrued but unpaid interest effective upon such conversion. All shares
of Preferred Stock issued upon conversion of this Note shall be entitled to all
accretion from the Original Issuance Date (as defined in the Certificate of
Designation), as provided in Section 4 of the Certificate of Designation.

     (b) If at the time of conversion of this Note into shares of Preferred
Stock there are insufficient authorized shares of Preferred Stock to permit
conversion of this Note in full, then the Maker shall take all corporate action
necessary to authorize a sufficient number of shares of Preferred Stock to
permit such conversion in full. No fractional shares of the Preferred Stock will
be issued upon conversion of this Note. In lieu of any fractional share to which
the Holder would otherwise be entitled, the Maker will pay to the Holder in cash
the amount of the unconverted principal balance of this Note that would
otherwise be converted into such fractional share. Upon conversion of this Note,
the Holder shall surrender this Note, duly endorsed, at the principal offices of
the Maker or any transfer agent of the Maker. At its expense, the Maker will, as
soon as practicable thereafter, issue and deliver to such Holder, at such
principal office, a certificate or certificates for the number of shares to
which such Holder is entitled upon such conversion, together with other
securities and property to which the Holder is entitled upon such conversion
under the terms of this Note, including a check payable to the Holder for any
cash amounts payable as described herein. Upon conversion of this Note and
payment of interest and for fractional shares as provided above, the Maker will
be forever released from all of its payment obligations and liabilities under
this Note with regard to that portion of the principal amount being converted.

     4.  Payments.  This Note may be prepaid in whole or in part at any time or
from time to time without premium or penalty after the earlier of the Negative
Vote Date and December 31, 2002. Any voluntary or mandatory prepayment of this
Note shall be applied first to the payment of interest accrued and unpaid on
this Note and second to the payment of principal. All payments by the Maker
under this Note shall be made without set-off, defense or counterclaim and be
free and clear and without any deduction or withholding for any taxes or fees of
any nature whatever, unless the obligation to make such deduction or withholding
is imposed by law. The principal balance and all accrued but unpaid interest on
this Note shall be paid in full on the Maturity Date.

     5.  Waiver.  No delay or omission on the part of the Holder in exercising
any right under this Note shall operate as a waiver of such right or of any
other right of the Holder, nor shall any delay, omission or waiver on any one
occasion be deemed a bar to or waiver of the same or any other right on any
future occasion. The Maker hereby forever waives presentment, demand,
presentment for payment, protest, notice of protest, notice of dishonor of this
Note and all other demands and notices in connection with the delivery,
acceptance, performance and enforcement of this Note.

                                        2
<PAGE>

     6.  Negative Covenants.  Without the prior written consent of the holders
of a majority in principal amount of the Notes then outstanding, the Maker shall
not, and will not permit any of its subsidiaries to (a) incur any indebtedness
for money borrowed, other than (i) indebtedness under the Notes, (ii)
indebtedness not in excess of $20 million in principal amount outstanding
(including without limitation under any existing lines of credit as of the date
of this Note) at any time and (iii) indebtedness the net proceeds of which are
used to repay the Notes in their entirety, (b) grant any liens, pledges or
encumbrances (collectively, "Liens") on any of its assets or permit any Liens to
exist other than (i) statutory liens or mechanics liens or non-consensual liens
that do not materially impair the value of the assets and (ii) Liens granted to
secure indebtedness permitted under clause (a) above, (c) sell any material
assets for consideration in excess of $10 million in the aggregate other than
sales of inventory in the ordinary course of the Maker's business or (d) pay or
declare any dividend or distribution (other than dividends or distributions by
wholly owned subsidiaries of the Maker to the Maker).

     7.  Event of Default.

     (a) The Maker agrees that: (i) upon the failure to pay when due the
principal balance and accrued interest hereunder; (ii) if the Maker (1)
commences any voluntary proceeding under any provision of Title 11 of the United
States Code, as now or hereafter amended, or commences any other proceeding,
under any law, now or hereafter in force, relating to bankruptcy, insolvency,
reorganization, liquidation, or otherwise to the relief of debtors or the
readjustment of indebtedness, (2) makes any assignment for the benefit of
creditors or a composition or similar arrangement with such creditors, or (3)
appoints a receiver, trustee or similar judicial officer or agent to take charge
of or liquidate any of its property or assets; (iii) upon the commencement
against the Maker of any involuntary proceeding of the kind described in
paragraph (ii); (iv) upon the acceleration of any other indebtedness of the
Company for borrowed money in excess of $1,000,000; (v) upon the rendering of a
judgment or judgments involving an amount in excess of $1,000,000 and such
judgment or judgments shall not have been vacated, discharged, stayed or bonded
pending appeal within 30 days from the entry thereof; (vi) if the Maker breaches
Section 5.6 of the Purchase Agreement; (vii) if Maker breaches Section 6 of this
Note; (viii) upon a Change of Control; (ix) if prior to the Special Meeting, the
Maker shall issue, or agree to issue, in a transaction exempt from registration
under the Act shares of Common Stock, or Common Stock equivalents, in an amount
in excess of 15% of the shares of Common Stock then outstanding; or (x) if the
Maker's Board of Directors withdraws or adversely modifies its recommendation
relating to the Proposal in accordance with Section 5.6 of the Purchase
Agreement (any of (i) through (x), an "Event of Default"), all unpaid principal
and accrued interest under this Note shall become immediately due and payable
without presentment, demand, protest or notice of any kind.

     (b) Upon the occurrence and continuance of an Event of Default, the Holder
shall have all the rights and remedies under the Uniform Commercial Code of the
State of New York.

     8.  Amendment.  None of the terms or provisions of this Note may be
excluded, modified or amended except by a written instrument duly executed by
the Holder and the Maker expressly referring to this Note and setting forth the
provision so excluded, modified or amended.

     9.  Costs.  If action is instituted to collect on this Note, the Maker
promises to pay all costs and expenses, including reasonable attorney's fees,
incurred in connection with such action.

     10.  Governing Law.  This Note shall be governed by, and construed in
accordance with, the laws of the State of New York.

     11.  Notices.  All notices hereunder shall be given in writing and shall be
deemed delivered when received by the other party hereto at the address set
forth in the Purchase Agreement or at such other address as may be specified by
such party from time to time in accordance with the Purchase Agreement.

     12.  Transferability.  This Note may not be transferred by the Holder
except in accordance with Section 7 of the Purchase Agreement.

                  [Remainder of Page Intentionally Left Blank]

                                        3
<PAGE>

     This Note shall be binding upon the successors or assigns of the Maker and
shall inure to the benefit of the successors and assigns of the Holder.

                                          PROXIM CORPORATION

                                          By:
                                            ------------------------------------
                                            Name:
                                            Title:

                                        4