Using rubber duckies indoors?

[Benzman66]

Hopefully someone out there can give me an idea. My mom lives in an apartment complex that does not allow outside antennas. I bought her a Pro-2096 mainly to listen to Allegheny County, but also Westmoreland 800 system. Using the stock antenna just won't being in the 800 mhz as it is too far away, so I came up with the idea of using a magnet mount base and putting it on a cookie sheet, I then used a duplex connector to attach the stock telescopic and a RS 800 mhz rubber duck to it. (Sometimes just the standard rubber and 800 rubber ones)

It works well, but still weak on the 800 mhz, so I took the splitter off and just connected the 800mhz directly to the mag mt and got PERFECT reception. It seems by using that splitter it cuts down a lot on the rf gain for 800 mhz.

Using just the 800mhz antenna is fine, but cuts down on the regular frequencies reception, which is why I like using both antennas. Does anyone have any ideas how to use both without losing the gain?

[gewecke]

Go to radio shack and pick up one of their all band mag mount antennas and slap it on your frig,file cabinet,freezer. Problem solved!
N9ZAS

[FleetAdmiral]

I would have to say that a frequency independent antenna (which is a rather deceptive term used by the IEEE to classify antennas) or a broad band antenna would be the easiest approach to the problem as mentioned by gewecke, since some advanced RF topics arise when trying to combine multiple antennas together.

But if you’re determined to combine the two antennas that you have together here are some things you might want to consider just off the top of my head.

First of all you need to know what the input impedance of the antennas you are trying to connect together are along with the input impedance of the scanner because impedance mismatch can cause RF signal reflection which in turn reduces the amount of signal that your scanner can receive.

For example if you have a 50 ohm antenna and are using 300 ohm TV cable to make the connection from the antenna to the scanner that impedance mismatch is going to cause RF reflection and reduce the amount of signal received by the scanner a lot.

Now since it sounds like you are using standard store bought equipment the individual antennas impedance by themselves should matched your scanner however I believe the interconnection between the two antennas is what is giving you problems since input impedance to a antenna is a function of the antennas physical structure and by adding an interconnection thru that duplex connector you are most likely changing the input impedance of the antenna which in turn causes an impedance mismatch that causes RF reflection and signal reduction which you clearly saw when you had the antennas hooked together compared to when they were not hooked together.

Now I don’t consider myself an expert on this subject but one suggestion that you might possibly want to consider is replacing the duplex connector with a power divider (aka signal splitter) which is designed to connect to your antennas input impedance and scanner input impedance which I am assuming is 50 ohms and divide the power evenly between the two antennas to the scanner.

Now you might be wondering, wait a minute why would I use a power divider when I’m not transmitting anything, well as it turns out antennas are reciprocal devices so they transmit the same way that they receive thus the power divider is actually combining the signals received from each antenna together on the scanner input connection.

Now here is a homebrew hack you could try, I believe most cable TV power dividers are designed to operate up to 1 GHz but there impedance is 75 ohms which means that there will be some impedance mismatch and RF reflection but it should work reasonably well as long as it’s the only 75 ohm mismatch across the whole antenna configuration meaning that 50 ohm wire and connectors are used everywhere else for a quick and dirty test depending upon the type of mounting hardware you currently have around the house. I bring this homebrew hack up because I have found most local cable TV providers at least in my area willingly give out TV splitters for free when requested and there relatively easy and cheep to obtain when you can’t get one for free.

But the most elegant method would be to buy a splitter that is designed to operate at your antennas and scanners input impedance along with have the required wire connections since your most likely working with a BNC or SMA connector rather than a coax connection.

So fundamentally I believe if you had a splitter that was designed for your antennas and scanner input impedance you would be relatively good to go and everything would work for you.

But there is also another point you might want to consider which arises from the fact that when you have two or more antennas connected together you create what is called an antenna array which has unique and interesting properties that could cause problems inadvertently. To quickly summarize without going into any depth on this subject, the length of the wire going to each antenna, how the power is divided between the two antennas by the splitter, the spacing between the two antennas and the structure of the antennas connected together can modify how the overall antenna as a whole will function.

Now antenna designers oftentimes manipulate these variables to create really awesome antenna arrays that can transmits and receive signals from a target location on earth from space per say but an unlucky combination by random construction could result in your antenna array picking up a signal from the north very well but nothing at all in every other direction at a particular frequency.

Now while I believe the chances of this happening are minimal but to be safe sometimes people will incorporate RF filters for the operational band that the antenna should receive on to reduce the chance that one antenna will interfere with the other as a precautionary measure.

Personally I think you should just move the antennas around until the array functions correctly if an odd directivity issue occurs for a given frequency since RF filter design and impedance matching is not something I would consider to be a fun filled afternoon activity.

The last thing I can think to mention is the fact that being that the refrigerator is made of metal so special care should be taken to avoid affecting the antennas near field since metal in close proximity to a antenna will change how the antenna transmits and receives along with change the input impedance of the antenna. Now since most rubber duckies and mono pole antennas have no pattern in there vertical direction a metal plate under the antenna should actually work well and not change how the antennas operates too much but you would not want to build a metal frame per say to hold the two antennas upright and together in the middle of each antenna as metal would change how the antenna operates.

that’s about all I can think of,

Hopefully this is useful information.

[Benzman66]

Thanks for the replies!
I did try the all band mag mount antenna on the cookie sheet, but the 800mhz just can't make the trip with that, which is why I was messing with the 800 mhz rubber duck.

Fleet. I think you hit the nail on the head with the impedance issue. The mag mount base I was using has the female base BNC, so I got a 2 female BNC to 1 BNC male splitter and used that, then used the 800mhz and stock telescopic on the female ends. The base has about 5 feet of what I would think is RG58 cable and a BNC connector at the end to the scanner, so all connections from start to finish are all BNC and factory stock, no handwire, makeshift connections. I have a bunch of the coax splitters laying around at my house, so I might have to give that a try too. At my home, I have a outside antenna, but unfortunately, for my mom in the apartment, no can do.

Any other ideas or input will be appreciated. Thanks again.

[N1BHH]

Try repositioning your antenna, because that makes a difference. Also, distance should be part of the equation as well as building materials. Some building materials are favorable to reception, but they all have their absorption factors. If you are in a wood structure with paper backed insulation you shouldn't have too much trouble. Whereas if the structure is metallic, then you will have a problem. I currently have a couple of my mag mount antennas on my air conditioner and they do well. I used to live in a third floor apartment and had one on my refrigerator, which worked quite well.