New Adcock RDF idea

LarrySC · #1 (**permalink**) 06-19-2009, 08:19 AM

Over the years several Mfg's have made VHF RDF systems like the Regency Marine Polaris and I think Ramsey still makes one but this idea seems better. The idea is to use NMO hardware on small mag mounts. Supplied this way the consumer could buy whatever antennas he needed based on how much $ he could put into it. The antennas usually are proprietary which offers no option to antenna selection. I would be interested in an Adcock RDF type antenna if it had this feature. I tried to locate a good Polaris system but being 200 miles from the coast made it very hard. Ebay was out because of size of the Polaris antenna being shipped. SO, with hopes of someone reading this someway connected to the Mfg'ing of antenna systems is a long shot. Remember, lots of antennas are just made to SELL. A few select antennas are made to WORK. Best regards, Larry

DaveNF2G · #2 (**permalink**) 06-19-2009, 10:45 AM

The accuracy of the system would be affected by antenna selection. All antennae must be identical, and their size and therefore radiation patterns must be calibrated to the element spacing.

LarrySC · #3 (**permalink**) 06-20-2009, 01:10 PM

None of those issues were disputed. Antenna selection would the the option of the buyer - not the seller. They would of course all be the same. The element spacing is controlled by the Mfg's assembly of cable and hardware. The idea is: I would rather use VHF 3db Gain ant's, or something else, in place of 1/4wave whips supplied by the Mfg. There are at least 200 antennas on the commerical market that use NMO. Thanks for your reply, Larry

DaveNF2G · #4 (**permalink**) 06-21-2009, 01:16 AM

A gain antenna has gain because its radiation pattern is altered. A gain antenna will affect the system's DF accuracy.

prcguy · #5 (**permalink**) 06-21-2009, 12:36 PM

I don't think a gain antenna has anything to do with the accuracy however the spacing between antennas and precise length of coax for a specific band does matter.
OAR sold a mag mount box with 4 whips for its mobile direction finding equipment so there are other designs out there.
prcguy

ab3a · #6 (**permalink**) 06-22-2009, 10:07 AM

Quote:

Originally Posted by DaveNF2G

A gain antenna has gain because its radiation pattern is altered. A gain antenna will affect the system's DF accuracy.

For those who don't know where the gain comes from (I presume Dave already knows this), Imagine a normal quarter wave vertical antenna: It has a doughnut shaped far-field radiation pattern. Now imagine a gain antenna: It has a pattern of a flattened doughnut.

Dave, I understand your misgivings. Many gain antennas, such as a 5/8 wave, depend strongly upon having a good ground plane. Installing more than one of these things on a car roof could make for a less than omnidirectional pattern. But that would be the case with 1/4 wave verticals too. I'll agree that with gain antennas such as a co-linear array, there will be side lobes above and below the main lobe. This means that unless your signal source and you are relatively level, you might run in to trouble figuring out where something is coming from.

The concept could work. But you'd have to be very careful to avoid antennas with more than one element, and you'd need to be certain you've got a good ground plane underneath it. This is good advice for any Adcock array, not just one with gain.