Another newbie with antenna question

[Markb]

Hi All.

I just picked up a Dorne & Margolin DMC-120 off of EBay for $120-ish. Here's the link with pics:

http://tinyurl.com/cuso5yp



It was red tagged (non-repairable) as the auction stated. Looks like it got dropped from the top of a building it something. I'm thinking I can probably fix it, though.
Does anyone know if there is a place to order parts for these? I may be able to get is back to its original luster without, but new parts would be cool.

Here are the problems I've found:
The wire/cable that runs around the reflector (?) is broken. I'm thinking I can solder that back together or maybe run some wire around in its place. The only other thing that I can see is that one of the springs for the reflector poles is missing. I don't know if that's significant for RX or not. I'll probably just have to test it out once I make up a cable.

Last question for now: do I need a preamp to be able to get good RX with a scanner?

Thanks!
Mark

[rfking123]

Originally Posted by Markb

Hi All.

I just picked up a Dorne & Margolin DMC-120 off of EBay for $120-ish. Here's the link with pics:
View item:
Dorne and Margolin sitcom antenna. Type DMC120-3, used, fair condition.


It was red tagged (non-repairable) as the auction stated. Looks like it got dropped from the top of a building it something. I'm thinking I can probably fix it, though.
Does anyone know if there is a place to order parts for these? I may be able to get is back to its original luster without, but new parts would be cool.

Here are the problems I've found:
The wire/cable that runs around the reflector (?) is broken. I'm thinking I can solder that back together or maybe run some wire around in its place. The only other thing that I can see is that one of the springs for the reflector poles is missing. I don't know if that's significant for RX or not. I'll probably just have to test it out once I make up a cable.

Last question for now: do I need a preamp to be able to get good RX with a scanner?

Thanks!
Mark

Where is the link??

[Markb]

Sorry. Fixed it in the original post.

[prcguy]

As long as the internal parts are not burned up the problems you describe are not that bad to fix. The reflector wire is not too critical, just keep it the same length as the original.

Any new part no matter how small will cost you much more than you paid for the entire antenna.
prcguy

[blaze]

Assuming there's nothing wrong internally, you should be OK replacing/reconnecting the wire around the reflector, at least for receive-only use.

If the antenna is even approximately working, you shouldn't need a preamp with a modern receiver (assuming you're in a footprint and have a reasonably good view to the bird).

Since you already own it, you have nothing to lose by trying.

[Markb]

Thanks for the info guys. I have a few ideas on how to fix it. We'll see how it goes. Hopefully I'll have a cable within the next week and take her for a spin.
Matt, I printed out your guide and I'm using that to get started. Thanks for making that available.
I'm in Southern California, near Disneyland, so I should be able to point the thing in the right direction.
Are the birds always broadcasting a carrier, or is there something to tune into relatively quickly so I can verify whether it works?

[Rt169Radio]

What kind of antenna is that exactly? Tracking antenna?

[blaze]

Originally Posted by Markb

Thanks for the info guys. I have a few ideas on how to fix it. We'll see how it goes. Hopefully I'll have a cable within the next week and take her for a spin.
Matt, I printed out your guide and I'm using that to get started. Thanks for making that available.
I'm in Southern California, near Disneyland, so I should be able to point the thing in the right direction.
Are the birds always broadcasting a carrier, or is there something to tune into relatively quickly so I can verify whether it works?

When the birds are idle they transmit noise across their passband. This is hard to detect by ear but very easy to spot if your receiver has a spectrum display; see Some Example UHF Satellite Downlink Spectrum Plots .

Otherwise you'll want a reasonably continuous signal to test with. Over the Pacific, the 269.750 transponder usually has an Asian TV station's audio carrier going through it (set your receiver to wideband FM, tune to about 269,745, and don't worry, it's normal for it to be distorted).

Also over the Pacific, 250.550 often is transmitting a narrow (and easy to spot) carrier.

What are you using for a receiver?

[blaze]

Originally Posted by Rt169Radio

What kind of antenna is that exactly? Tracking antenna?

No, it's a fixed, portable folding tactical satcom antenna, designed specifically for military UHF TACSAT use. It has about 9db of gain and a fairly wide beamwith that allows it to be aimed by hand. Because the TACSAT satellites are geosynchronous, tracking antennas aren't needed.

[Markb]

Originally Posted by blaze

What are you using for a receiver?

I have several options. I have a Uniden BC-780 & BCD-396T, a Kenwood TH-F6 Triband amateur handheld and a handful of RTL sticks. Looks like the RTL's will be the way to go, for starters anyway.

[Markb]

Okay. I just went to Radio Shack and picked up a short pigtail to connect the antenna to an RTL dongle. The spectrum display did show many of the characteristic plateaus and I was able to hear the Asian TV audio reasonably well with 1 caveat - I had to manipulate the BNC connector to lower the noise floor to pull out the signals. A closer look of the BNC connector showed that the small metal "plates" surrounding the plastic core were gone! Now I need to see if I can replace it. It looks as though the housing is welded or glued, so I may not be able to get in without damaging the whole thing.
In another note :



I was able to reconnect the chain in the core of the reflector wire and sutured the wire with some small gauge magnet wire. Not too good for the Afghan mountains or the South American jungles, but good enough for Orange County.

[prcguy]

I always thought D&M made a bad choice in putting a difficult (impossible?) to replace BNC male chassis connector on their satcom antennas. If I had the same problem I would consider soldering a good BNC double female adapter to whats left of the connector you describe. It sounds like your connector center pin is ok and all you need to do is improve the outer shield connection.

If you can find a Teflon insulated, silver plated double female BNC it will take solder very easily and resist heat from soldering. You could file the plating off the end of the male connector on the antenna to expose the bare brass, insert the double female adapter and run a bead of solder around the outer end of the male connector to the body of the double female using a solder gun.

Since you would be soldering at the very outer end of the male connector and there is very poor heatsinking from that area to the rest of the connector and you should be able to solder it quickly without damaging any rubber parts of the connector.
prcguy

[Markb]

That's a great idea! That's probably what I'll do. The pigtail I bought is BNC to SMA and I'll probably use the SMA more than anything. Might just solder the pigtail on.
I spent a little more time playing around with it tonight and I used a small clamp to provide a little pressure on the connector and it seemed to work well. Gave me a chance to play around a little more.
Maybe the folks at D&M are smarter than we give 'em credit for.......you make them too easy to fix, you don't sell as many.

Mark

[blaze]

Don't open the unit if you can possibly avoid it. These antennas (Trivec and D&M) are generally filled with a nasty milspec polyurethane foam for weather- and shock- proofing. The stuff is messy and almost impossible to work around. So if you can make it work with a clamp on the connector, however, precariously, that's preferable to the almost certain destruction that will come from opening the thing up.

[Markb]

Wow. Thanks for that. I'll just stick with a clamp or maybe tie some wire to hold it in place and eventually solder the bounce BNC in.

Hearing Portuguese on 262.425 CONUS as we speak.