<PAGE>   1

                                                                   Exhibit 10.6



NEITHER THIS CONVERTIBLE PROMISSORY NOTE NOR THE VOTING COMMON STOCK INTO WHICH
IT IS CONVERTIBLE HAS BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, OR ANY APPLICABLE STATE SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED
FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF A REGISTRATION STATEMENT IN
EFFECT WITH RESPECT TO THE SECURITIES UNDER SUCH ACT AND APPLICABLE LAWS OR
SOME OTHER EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF SUCH ACT AND
APPLICABLE LAWS OR AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT SUCH
REGISTRATION IS NOT REQUIRED.


                            PINNACLE TOWERS IV INC.
                          CONVERTIBLE PROMISSORY NOTE

$9,600,000                                                      August 17, 2000


         PINNACLE TOWERS IV INC., a Florida corporation (the "Company"), the
principal office of which is located at 301 North Cattlemen Road, Suite 300,
Sarasota, Florida 34232, for value received hereby promises to pay to Pinnacle
Towers Inc., or its registered assigns, the sum of Nine Million Six Hundred
Thousand Dollars ($9,600,000), or such lesser amount as shall then equal the
outstanding principal amount hereof and any unpaid accrued interest hereon, as
set forth below. The outstanding principal under this Note shall be due and
payable in full within 30 days of the date demand is made therefor by the
Holder. Demand under this Note shall be given by the Holder to the Company by
written notice thereof in accordance with Section 11 below. Payment for all
amounts due hereunder shall be made at the Company's option by either wire
transfer or by mail to the registered address of the Holder.

         The following is a statement of the rights of the Holder of this Note
and the conditions to which this Note is subject, and to which the Holder
hereof, by the acceptance of this Note, agrees:

         1.       Definitions. As used in this Note, the following terms,
unless the context otherwise requires, have the following meanings:

                  (a)      "Company" includes any corporation which shall
succeed to or assume the obligations of the Company under this Note.

                  (b)      "Holder," when the context refers to a holder of
this Note, shall mean any Person who shall at the time be the registered holder
of this Note.



<PAGE>   2

                  (c)      "Person" means any individual, Company, partnership,
joint venture, trust, unincorporated organization or government or any agency
or political subdivision thereof.

         2.       Interest. Commencing on September 30, 2000, and on each
December 31, March 31, June 30 and September 30 thereafter until all
outstanding principal and interest on this Note shall have been paid in full,
the Company shall pay interest at the rate of thirteen percent (13%) per annum
(the "Initial Interest Rate") on the principal of this Note outstanding during
the period beginning on the date of issuance of this Note and ending on the
date that the principal amount of this Note becomes due and payable. In the
event that the principal amount of this Note is not paid in full when such
amount becomes due and payable, interest at the same rate as the Initial
Interest Rate plus two percent (2%) shall continue to accrue on the balance of
any unpaid principal until such balance is paid.

         3.       Conversion.

                  3.1      Conversion. Any Holder of this Note has the right,
at the Holder's option (the "Option"), at any time prior to payment in full of
the principal balance of this Note, to convert this Note, in accordance with
the provisions of Section 3.2 hereof, in whole or in part, into fully paid and
nonassessable shares of the Company's Voting Common Stock, par value $0.001 per
share or Nonvoting Common Stock, par value $0.001 per share (the "Common
Stock"), at the option of the Holder from time to time. The number of shares of
Common Stock into which this Note may be converted ("Conversion Shares") shall
be determined by dividing the aggregate principal amount of this Note together
with all accrued interest to the date of conversion elected to be converted by
the Holder by the Conversion Price (as defined below) in effect at the time of
such conversion. The initial Conversion Price shall be equal to $25 (the
"Conversion Price").

                  3.2      Notice of Conversion Pursuant to Section 3.1. Before
the Holder shall be entitled to convert this Note into shares of Common Stock,
it shall surrender this Note at the office of the Company and shall give
written notice to the Company at its principal corporate office of the election
to convert all or a portion of the same pursuant to this Section 3 ("Notice of
Conversion"), and shall state therein the amount of the Note to be converted
and the name or names in which the certificate or certificates for shares of
Common Stock are to be issued. Such conversion shall be deemed to have been
made immediately prior to the close of business on the date of such surrender
of the Note, and the Person or Persons entitled to receive the shares of Common
Stock issuable upon such conversion shall be treated for all purposes as the
record holder or holders of such shares of Common Stock as of such date.



                                       2
<PAGE>   3


                  3.3      Mechanics and Effect of Conversion. No fractional
shares of Common Stock shall be issued upon conversion of this Note. In lieu of
the Company issuing any fractional shares to the Holder upon the conversion of
this Note, the Company shall pay to the Holder the amount of outstanding
principal that is not so converted, such payment to be in the form as provided
below. Upon the conversion of this Note pursuant to Section 3.1 above, the
Holder shall surrender this Note, duly endorsed, at the principal office of the
Company. At its expense, the Company shall, as soon as practicable thereafter,
issue and deliver to such Holder at such principal office a certificate or
certificates for the number of shares of such Common Stock to which the Holder
shall be entitled upon such conversion (bearing such legends as are required by
applicable state and federal securities laws in the opinion of counsel to the
Company), together with any 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 above and
a replacement Note representing any amount of the Note not converted. Upon the
complete conversion of all of this Note, the Company shall be forever released
from all its obligations and liabilities under this Note, except that the
Company shall be obligated to pay the Holder, within ten (10) days after the
date of such conversion, any interest accrued and unpaid or unconverted to and
including the date of such conversion, and no more.

         4.       Prepayment. Prepayment of the principal of this Note is
permitted, in whole or in part, without premium or penalty of any kind;
provided the Company provides the Holder with thirty (30) days' prior written
notice (unless notice is waived in writing by the Holder) of its intention to
prepay the principal of this Note, in whole or in part, during which time the
Holder may exercise the Option by delivering to the Company the Notice of
Conversion.

         5.       Conversion Price Adjustments.

                  5.1      Adjustments for Stock Splits and Subdivisions. In
the event the Company should at any time or from time to time after the date of
issuance hereof fix a record date for the effectuation of a split or
subdivision of the outstanding shares of Common Stock or the determination of
holders of Common Stock entitled to receive a dividend or other distribution
payable in additional shares of Common Stock or other securities or rights
convertible into, or entitling the holder thereof to receive directly or
indirectly, additional shares of Common Stock (hereinafter referred to as
"Common Stock Equivalents") without payment of any consideration by such holder
for the additional shares of Common Stock or the Common Stock Equivalents
(including the additional shares of Common Stock issuable upon conversion or
exercise thereof), then, as of such record date (or the date of such dividend
distribution, split or subdivision if no record date is fixed), the Conversion
Price of this Note shall be appropriately decreased so that the number of
shares of




                                       3
<PAGE>   4

Common Stock issuable upon conversion of this Note shall be increased in
proportion to such increase of outstanding shares.

                  5.2      Adjustments for Reverse Stock Splits. If the number
of shares of Common Stock outstanding at any time after the date hereof is
decreased by a combination of the outstanding shares of Common Stock, then,
following the record date of such combination, the Conversion Price for this
Note shall be appropriately increased so that the number of shares of Common
Stock issuable on conversion hereof shall be decreased in proportion to such
decrease in outstanding shares.

                  5.3      Notices of Record Date, etc. In the event of:

                           (a)      Any taking by the Company of a record of
the holders of any class of securities of the Company for the purpose of
determining the holders thereof who are entitled to receive any dividend (other
than a cash dividend payable out of earned surplus at the same rate as that of
the last such cash dividend theretofore paid) or other distribution, or any
right to subscribe for, purchase or otherwise acquire any shares of stock of
any class or any other securities or property, or to receive any other right;
or

                           (b)      Any capital reorganization of the Company,
any reclassification or recapitalization of the capital stock of the Company or
any transfer of all or substantially all of the assets of the Company to any
other Person or any consolidation or merger involving the Company; or

                           (c)      Any voluntary or involuntary dissolution,
liquidation or windingup of the Company, the Company will mail to the holder of
this Note at least seven (7) days prior to the earliest date specified therein,
a notice specifying:

                                    (i)         The date on which any such
record is to be taken for the purpose of such dividend, distribution or right,
and the amount and character of such dividend, distribution or right; and

                                    (ii)        The date on which any such
reorganization, reclassification, transfer, consolidation, merger, dissolution,
liquidation or windingup is expected to become effective and the record date
for determining shareholders entitled to vote thereon.

                  5.4      Reservation of Stock Issuable Upon Conversion. The
Company shall at all times reserve and keep available out of its authorized but
unissued shares of Common Stock solely for the purpose of effecting the
conversion of this Note such number of its shares of Common Stock as shall from
time to time be sufficient to effect the conversion of the Note; and if at any
time the number of authorized but unissued shares of Common Stock shall not be
sufficient to effect the




                                       4
<PAGE>   5

conversion of the entire outstanding principal amount of this Note, in addition
to such other remedies as shall be available to the holder of this Note, the
Company will use its best efforts to take such corporation action as may, in
the opinion of its counsel, be necessary to increase its authorized but
unissued shares of Common Stock to such number of shares as shall be sufficient
for such purposes.

         6.       Representations and Warranties of the Holder.

                  (a)      The Holder by its acceptance of this Note
acknowledges that it is aware that this Note and the shares of Common Stock
issuable to it by the Company upon conversion of this Note have not been
registered under the Securities Act of 1933, as amended ("Act"), or the
securities laws of any state or other jurisdiction.

                  (b)      The Holder warrants and represents to the Company
that it has acquired this Note, and, upon conversion of the Note, it will be
acquiring the Common Stock, for investment and not with a view to or for sale
in connection with any distribution of this Note or such Common Stock or with
any intention of distributing or selling this Note or such Common Stock.

                  (c)      The Holder has no right to demand that the Company
register this Note or the shares of Common Stock issued or issuable under this
Note.

         7.       Assignment. Subject to the restrictions on transfer described
in Section 11 below, the rights and obligations of the Company and the Holder
of this Note shall be binding upon and benefit the successors, assigns, heirs,
administrators and transferees of the parties.

         8.       Waiver and Amendment. Any provision of this Note may be
amended, waived or modified upon the written consent of the Company and the
Holder.

         9.       Transfer of This Note or Securities Issuable on Conversion
Hereof. With respect to any offer, sale or other disposition of this Note or
securities into which such Note may be converted, the Holder will give written
notice to the Company prior thereto, describing briefly the manner thereof,
together with a written opinion of such Holder's counsel, to the effect that
such offer, sale or other distribution may be effected without registration or
qualification (under any federal or state law then in effect). Promptly upon
receiving such written notice and reasonably satisfactory opinion, if so
requested, the Company, as promptly as practicable, shall notify such Holder
that such Holder may sell or otherwise dispose of this Note or such securities,
all in accordance with the terms of the notice delivered to the Company. If a
determination has been made pursuant to this Section 9 that the opinion of
counsel for the Holder is not reasonably satisfactory to the Company, the
Company shall so notify the Holder promptly after such determination has been
made. Each Note thus transferred and each certificate




                                       5
<PAGE>   6

representing the securities thus transferred shall bear a legend as to the
applicable restrictions on transferability in order to ensure compliance with
the Act, unless in the opinion of counsel for the Company such legend is not
required. The Company may issue stop transfer instructions to its transfer
agent in connection with such restrictions.

         10.      Treatment of Note. To the extent permitted by generally
accepted accounting principles, the Company will treat, account and report the
Note as debt and not equity for accounting purposes and with respect to any
returns filed with federal, state or local tax authorities.

         11.      Notices. Any notice, request or other communication required
or permitted hereunder shall be in writing and shall be deemed to have been
duly given if personally delivered or if (and then two business days after)
mailed by registered or certified mail, postage prepaid, at the respective
addresses of the parties as set forth herein. Any party hereto may by notice so
given change its address for future notice hereunder.

         12.      No Shareholder Rights. Nothing contained in this Note shall
be construed as conferring upon the Holder or any other Person the right to
vote or to consent or to receive notice as a shareholder in respect of meetings
of shareholders for the election of directors of the Company or any other
matters or any rights whatsoever as a shareholder of the Company; and no
dividends or interest shall be payable or accrued in respect of this Note or
the interest represented hereby or the Conversion Shares obtained hereunder
until, and only to the extent that, this Note shall have been converted.

         13.      Governing Law. This Agreement shall be governed by and
construed in accordance with the laws of the State of Florida, excluding that
body of law relating to conflict of laws.

         14.      Heading; References. All headings used herein are used for
convenience only and shall not be used to construe or interpret this Note.
Except where otherwise indicated, all references herein to Sections refer to
Sections hereof.



                                       6
<PAGE>   7

         IN WITNESS WHEREOF, the Company has caused this Note to be issued this
17th day of August, 2000.

                                         PINNACLE TOWERS IV INC.


                                         By
                                           ------------------------------------
                                                 Steven R. Day, Vice President

                                         Name of Holder:
                                                        -----------------------
                                         Address:
                                                 ------------------------------

                                                 ------------------------------




                                       7