Tom, Perhaps you could enlighten everyone who read these discussions with your success in modifying a DB224 from the 150-162 range to the 220-225 range. You are the first I have ever heard of having done this. I know I would be particularly interested in the harness transmission lines used and their lengths, and their characteristic impedance's. I would also be interested in the impedance measurements made of the dipole configuration after being modified, how you determined the appropriate coax transformation impedance's, and how you compensated for the positive and negative reactances which appear as the freq. is varied from the center freq. to the band edges in each transformation. Would you also include the instrumentation used for your designs? If you employed the use of a Smith Chart in your calculations can you provide a copy showing the impedance response curve vs. freq. for the range 220-225 MHz or whatever you measured? There are no "loops" on a DB224, only folded dipoles. Loops are circular, dipoles are linear.
Allan Crites WA9ZZU --- On Fri, 2/19/10, Tom Manning <[email protected]> wrote: From: Tom Manning <[email protected]> Subject: Re: [Repeater-Builder] DB224A new harness To: [email protected] Date: Friday, February 19, 2010, 9:15 PM Hello Allan As you note it would take a good deal of work to go from 154 to 220Mhz. Yes the loops would have to be shortened and the harness would have to be be reworked-shortened. This would be easier than changing to 144Mhz. I have done this twice in the past. 73 de Tom Manning ----- Original Message ----- From: allan crites To: Repeater-Builder@ yahoogroups. com Sent: Thursday, February 18, 2010 5:47 PM Subject: Re: [Repeater-Builder] DB224A new harness Tom I would certainly be interested in knowing just how a 150-162 MHz DB224 could be "easily be modified to cover 220". The dipoles would need to be physically shortened and the matching harness would need to be completely redesigned. What magic do you have in your bag of tricks to manage this? Allan Crites WA9ZZU --- On Thu, 2/18/10, Tom Manning <af...@bellsouth. NET> wrote: From: Tom Manning <af...@bellsouth. NET> Subject: Re: [Repeater-Builder] DB224A new harness To: Repeater-Builder@ yahoogroups. com Date: Thursday, February 18, 2010, 4:58 PM Hello Andrew and Norm If you are trying to figure out the lengths needed for 144-148Mhz I can measure what you need to know but not in a hurry. I have a DB products antenna that is cut for the two meter band that I ordered from DB Products about 10 years ago. What I need to know is the measurements for 220Mhz. I also have several DB224a's that have been removed due to problems. These could easily be modified to cover 220Mhz. 73 de Tom Manning, AF4UG ----- Original Message ----- From: NORM KNAPP To: Repeater-Builder@ yahoogroups. com Sent: Wednesday, February 17, 2010 10:35 PM Subject: RE: [Repeater-Builder] DB224A new harness The unknown coax type is/was VB-83 (35 ohm). The VB-8 is of course 50 ohm and the VB-11 is 75 ohm. I hope that is the info you needed. It would be great if you could figure out what the open stub is for and can we use that for adjustments. If you need more info, let me know. 73 Norm ____________ _________ _________ __ From: Repeater-Builder@ yahoogroups. com on behalf of allan crites Sent: Wed 2/17/2010 9:29 PM To: Repeater-Builder@ yahoogroups. com Subject: RE: [Repeater-Builder] DB224A new harness I tried to work backwards on a Smith Chart from the dipole to the 1/2 WL O.C. stub to determine a reason for the stub but it appears the unknown coax types are causing erroneous and confusing answers. Can you confirm the coax types shown on your diagram? a. --- On Wed, 2/17/10, NORM KNAPP <nkn...@twowayradio. net> wrote: From: NORM KNAPP <nkn...@twowayradio. net> Subject: RE: [Repeater-Builder] DB224A new harness To: Repeater-Builder@ yahoogroups. com Date: Wednesday, February 17, 2010, 3:57 PM I got the harnesses at no charge from Andrew. I contacted customer support and told them I had 3 fairly new DB224's (2 a models and a b model) that had water intrusion/contamina tion in the harness where they were supposedly seal by the factory. I offered pictures and they accepted. Upon seeing the photos they asked that I give them serial numbers and requested I send the defective harnesses to them. I did and they offered and I accepted replacement harnesses. The two db224's that I have (personally) are the A models. The B model belongs to my shop. I often end up with abandoned and or defective db224's and other antennas as I work for a two way land mobile dealer/shop. Unfortunately, I have yet to get any DB224e's or any DB304's, just tons of DB224a's and an occasional DB224b. Hats off to Andrew for shipping me free replacement harnesses. I am still gonna ScotchKote those moldings regaurdelss. If you look at the attached photoes from my initial post, you will see that the open 25" VB-8 (50 ohm) stub is 12.5" from the "T" where the top half and bottom halves of the harness come together, down the main feed to the hardline (or whatever you feed the antenna with). Last night I connected the Sitemaster up again and I noticed the the SWR will come down some when I firmly secure the harness to the mast with the metal tape. How far down remains to be seen. I will keep you posted. 73 and thanks! de N5NPO Norm
