dual db420 antenna

[domes]

I am considering a commercial application with a dual db420 (two db408's on one 20ft mast). for separate transmit and receive antennas. Multiple repeater, trunked. Manufacturer specs claim 30 db isolation which is about what you get mounting 2 antennas with about 20 ft vertical separation. Has anyone had any success with this application?

 

[freddaniel]

In most cases the dual DB408's are for dual transmit, with at least one isolator per transmitter per antenna. If a DB420 were used with a hybrid coupler, the same ERP would occur.

When using one for transmit and one for receive, the 30 dB isolation is usually not enough to reduce the transmitter noise and desense to work effective, without adding notch cavities, that add additional losses. Actually, 30 dB isolation is provided by just 3 ft between the nearest dipoles on UHF. With 10 ft between the nearest dipoles the isolation would be about 50 db, and 20 ft would yield 60 dB. The rule of thumb from the old school is to separate the UHF Tx and Rx by at least 15 ft vertical, and use a good receiver preselector to avoid problems under almost any condition.

This is an engineering question. Therefore, it depends upon power out, frequency separation [3 or 5 MHz], actual receiver isolation requirements, and transmitter noise specifications. In addition, you state it is a "multiple repeater trunked" system, so the specs of the proposed receiver and transmitter multicouplers are also important. In short, it may work with just 30 dB isolation, but it is just as likely it may not.
To find out, build the system in the shop and test it local. Be sure to measure the desense using the Motorola method. There is lots of information available on the internet regarding the amount of isolation required for proper operation.

 

[domes]

Thanks for your response.These are 450-470 channels with 5 MHz pairs. I have the IM calculations. This is actually an existing system with adequate vertical separation with cavity/isolator combiner and filtered RX multi-coupler that I want to relocate to another site where I can only mount 2 antennas at the same height. I'm trying to decide if mounting separate tx & rx antennas on 3ft arms on 2ft face (8ft horizontal separation) or the db420D which I believe is actually two 10ft db408's stacked on a 20ft mast with 2 leads. It may have some special phasing engineered in the design. It probably has about 3ft between the top & bottom loops of the 2 antennas as you mention in your response. Sometimes the tower between tx & rx antennas can add isolation and sometimes it can actually couple the antennas. I'm thinking that the 420D spec'd isolation would not be affected by the tower?????

I looked at this subject last year but got involved in other sites and put this project on the back burner. I guess I may have to bite the bullet and order in an antenna to play with.

 

[gtriever]

We've used DB420Ds for years. Without going into a lengthy response, my recommendation based on that experience would be to vertically separate the Tx and Rx antennas.

 

[freddaniel]

I agree with "gtriever" above. The reason is 30 dB isolation can be had between two dipoles with 8 ft of horizontal separation. However, because they are gain antennas, you would need to reduce the isolation by the total gain of the two antennas. Therefore, the isolation would be less than 15 dB overall.

Use the DB420D and if required, add Rec notch filters to eliminate any desense caused by overload, or Tx notch filters to eliminate any transmitter broadband noise.

As a last thought, if you have 20 ft vertical to mount a DB420 antenna, consider mounting one or two DB404 antennas on a 20 ft pipe. Each 404 has 3 dB less gain, but the smaller size of each 404 will yield you an extra 10 dB of isolation. In other words, a 408-404 combo would provide 40 dB isolation, and a 404-404 combo would provide 50 dB isolation. Depends if you need all the gain the 408 provides on either Tx or Rec. Do you have lots of handhelds or not?