P1225 mounting external antenna

oz_com · Post by **oz_com** » Fri Apr 06, 2007 6:09 pm

I would like to mount an external antenna on my P1225. I have a MX-threaded adapter, but this radio does not have a "grounding ring" at the antenna connector.

I'm not real strong on electronics, but my idea was to run a stranded 18-gage wire from the antenna adapter to the ground plate on the underside where the battery connects.

After a series of trials, what I observed was:

- The external antenna does put out some unquantifiable amount of RF, as evidenced by a simple diode-based meter;

- However, it puts out *more* RF if I disconnect the ground wire altogether;

- While I am running the setup through a wattmeter, it measures only 1 watt out, then jumps up to 2 watts when I remove that ground wire;

- Curiosly, I got the same results with one of my HT600s, which I know puts out a full 5 watts when mounted in its Converta-com;

- And yes, the battery is good, the threaded connection is tight, and the radio does otherwise put out enough RF to peg the hand-held meter when used with its rubber ducky.

Obviously, I do not know the best practices in improvising an external antenna, though I have read posts on the Batboard where the late Monty briefly alluded to a special harness he made for the purpose of grounding external antennas.

Can anyone make suggestions as to the theory involved, what kind of implementation "ought" to work, what kinds of wire should I use for this purpose, etc.?

Tim

HLA · Post by **HLA** » Fri Apr 06, 2007 7:42 pm

if it's recieve only then it probablly won't make a difference on the ground but if you have to transmit then i'd find a mobile to use.

oz_com · Post by **oz_com** » Sat Apr 07, 2007 5:43 am

I'm going to bump myself here, with a bit more info. Another post on the Batboard describes what I'm attempting to do:

http://batboard.batlabs.com/viewtopic.php?t=23300

In that post, one of the members describes a commercially-available adapter for the MT-1000 that "sort of screws in and has a grounding strap that somehow is wedged between the housing and the frame or shield."

Monty replied that he had made " 'Special' Adaptors for the HT600 / P200 / MT1000 series," unfortunately the images he posted are no longer available from the external server.

I have a hunch that the ground strap cannot be made from simply any wire, but I am not sure exactly what is called for (how wide or thick)... or, how to get just a short piece of it! I also imagine that it must be as short as possible; but how long is too long when dealing with UHF?

Anyone who has one of these adapters, your description will be most welcome!

To clarify HLA's question, this radio must receive and transmit. Also, because the external antenna has a stacked design, I believe it must be grounded in order to have gain on receive. (The antenna works quite effectively with my scanner, which has a grounded BNC-type post...)

Thanks for your thoughts,

Tim

llu727 · Post by **llu727** » Sat Apr 07, 2007 7:01 am

The P1225 does in fact have a grounding ring at the top of the radio housing. If you remove the antenna and look at it from the top, you had better see a metal ring. Internally, it makes a good ground contact with the chassis. With the proper adaptor you should read full power at that point and have no problem.

BJ

oz_com · Post by **oz_com** » Sat Apr 07, 2007 9:05 am

llu727 wrote:The P1225 does in fact have a grounding ring at the top of the radio housing. If you remove the antenna and look at it from the top, you had better see a metal ring.
BJ

Thanks for the tip. This is my first P1225, so I had nothing to compare it to when I purchased it and noticed that it doesn't have a grounding ring.

(There is a recessed flat under the antenna where such a ring might go, but none is present.)

And now I recall the words of the seller: "mint condition... has been professionally re-cased."

Obviously, that wasn't the case -- at least, not the case they used, if I may be pardoned a pun.

Any suggestions what I might try next to get a good solid ground? Is RF ground different in any way than electrical ground?

Tim

llu727 · Post by **llu727** » Sat Apr 07, 2007 9:44 am

that little ring not being there is your problem.. if it was recased, that explains it. Its only a couple penny part, to the best of my recollection, but you have to take the escutcheon off and the radio outof the housing to put it on. It makes a pretty solid RF ground to the metal frame. Dont have a manual here to look up part numbers, but even replacing the escutcheon, I would guess at less than $2.

Not sure which RF adaptor you are using, but with that one part it will work fine. Obviously the stud has to be inulated from the ground on the adaptor.

BJ

Will · Post by **Will** » Sun Apr 08, 2007 12:43 am

Hey Guys, I stand corrected, sorry.

llu727 · Post by **llu727** » Sun Apr 08, 2007 4:25 am

Sorry, I have to dispute that!

That info is correct for the GP300/350 series, but the P1225 does not have the RF Jack inside like the GP.

The P1225 uses the SP50 adapter. The SP50 has the same ground ring arrangement.

BJ

arlojanis · Post by **arlojanis** » Sun Apr 08, 2007 11:05 am

The above post is correct. The adaptor is part number HLN9087A and is for SP50 and P1225. Its outside connector is a BNC.