antenna help

[pocho]

I have a few questions for you all. As a result of 50 MPH winds yesterday, my Scantenna on my second floor roof is severely damaged; the top and bottom elements fell :roll: . On my scanner, I mainly moniter Airband, and soon, the 800 mhz. Is there an antenna that will give me excellent results in both bands (airband & 800mhz) :?: Should I just buy another Scantenna :?: Also, I've been wondering.. Is it possible to use two outdoor/rooftop antennas for one scanner, or will it overload it :?: One antenna would be designed for Airband, and the other antenna would be designed for 800 Mhz.
Any replies are appreciated.

Thanks,
pocho 8)

 

[Tweekerbob]

Sorry to hear about your loss.

I don't have much experience with these SCANTENNA devices so I am not sure which way you should go.

How far away will these 800MHz signals be? If they are distant, over say 20-25 miles away, I would invest in a good 800 MHz yagi (beam).

You can make an airband groundplane antenna which should have about the same performance as your Scantenna for a total cost of around $5-$10. (see some recent posts in this forum for more info)

If you elect to go the two antennas route vs. the one SCANTENNA (new), then DO NOT couple them together. You can either switch them manually either with a coax switch or by unplugging one and plugging in the other.

Also, for 800 MHz, if the stations you are trying to listen to are distant, make your next investment some good low loss coax such as LMR 400 or Belden 9913. Be sure to keep your coax lengths as short as possible. In addition, be sure to use N, TNC or BNC connectors on the 800 MHz side.

HTH

Ryan

 

[flyingwolf]

Out of curiosity why should I not couple the two anteanna's together? I understand you say I shouldnt but what is the reason I shouldnt?

 

[merlin-7]

someone correct me if I am wrong but I think the antenna should be 50 ohms if you connect 2 than it drops to 25 ohms (bad) plus if any of the coax is different lengths, that adds a whole new problem.

 

[merlin-7]

someone correct me if I am wrong but I think the antenna should be 50 ohms if you connect 2 than it drops to 25 ohms (bad) plus if any of the coax is different lengths, that adds a whole new problem.

sorry I got an echo....cyber skip?

 

[LarrySC]

For 800 and Air a flat top discone would work. In other freq ranges a discone would require the vertical element. I guess they should call that one a Discone-V. Good Luck, Larry

 

[Tweekerbob]

You do not want to couple both antennas together because they will cancel each other out on ALMOST every frequency. This is the very short answer. You will end up with worse performance than you would have with just one antenna.

I recommend the switch because it is a cheap and easy solution.

The other solution that would allow you to couple these two antennas together is called a diplexer. However, I havn't been able to find one that would suit your needs. This device isolates the two antennas from each other and virtually eliminates the interaction between the two.

The only solution I have found that would enable to you to sucessfully (to some degree) couple these two antennas is to put a high pass filter (500MHz cut off) in line with your 800 MHz antenna (probably install it at the feed point of the antenna) and a low pass filter (somewhere around 300 MHz) on your Air band groundplane. Then couple them with a high quality T connector and run the coax into the scanner.

Here is a website with such filters http://www.surplussales.com/Mini-Circuits/HighPassFilt.html.

This accomplishes virutally the same thing as a diplexer, albeit a bit more lossy with the added connectors/adapters and such, but should work.

But, it you decide to get another Scantenna or build yourself a groundplane, and then the 800 MHz signal goes on the air and you hear it no problem...Don't fix it if it ain't broke.

 

[loumaag]

You do not want to couple both antennas together because they will cancel each other out on ALMOST every frequency. This is the very short answer. You will end up with worse performance than you would have with just one antenna.

I would be interested in the long answer. Especially, on what principal do you base this? :?:

 

[pocho]

Thanks for the replies guys. The 800 mhz station is pretty close.. just a few miles away. Well, I'm thinking of just buying a single antenna for both airband and 800 Mhz. Would this be a good one? http://www.nevadaradio.co.uk/acatalog/scanmaster-b1300.html :wink:

 

[TeRayCodA]

Broken scantennas are easy to repair:

Drill out the rivets out of the broken parts,drill holes in the element(I am assuming it broke at the insulator,where they almost always break)

Use some 1 & 1/2" 10/32 stainless steel nuts/bolts,put it all back together,and, as an added reinforcement measure,get one of those 6 ft "bicycle flags",tear off the flag,run it along the Vhf lo element(longest element),and use some black electrical tape to tape the fiberglass rod along the element.

I have done this,as I live in "tornado alley"as high winds have totalled one of my scantennas before.