Monitenna/Scantenna, mounting, and...

[77monarch77]

Hi all,

I've been a scannist for years but, due to a disability, have just started to get serious about it. I recently picked up a "good" scanner... a Pro-95. It works well and the trunking feature is great. I've pretty much got the EDACS system figured out. The Motorola system... well... that's another story for another time. Since, like I said, I've gotten serious about the hobby, I was looking for a good base antenna. An elderly gentleman that just recently passed away had a Monitenna that his son removed from the roof of his house. All of this guy's gear was practically given away (naturally, I found out too late) and the antenna was all that remained. I acquired it for $10. It's in excellent condition.

Anyway, I have a few questions about mounting this antenna. First, the antenna is attached, fastened to, or integrally a part of an arm (of sorts). Looking at it, one end is the antenna end and the other is the mounting end. My intention is to mount this on a standard (stock) RS 1 1/4 mast. In doing so, won't the antenna end sag after a time? If it won't sag on it's own, it will sure get some help this winter with the addition of snow weight. [I live in Central New York State]. Would it be a good idea to add a gusset or brace (between the arm and the mast) to offset the weight differential caused by the snow load or time sag?

Second, I know that this is an omni-directional antenna, but in a strange sort of way it kind of looks like a "misfit" yagi. In other words, it looks like it may perform better if it was pointed (aimed) in the general direction of the tower facility which uses the frequencies that I monitor most often. Is this true or did my overactive mind just conjure up this "theory"? If it is true, and as far as aiming goes, which end should be pointed towards the transmitter? Height won't be a factor as I should be about 20ft. above my roofline when it's installed. Yes... it'll be very securely mounted and sufficiently guyed as well!

One more question... I was thinking of having a dedicated 800Mhz antenna just for trunking purposes. I was also thinking of making my own. I'd like to take a mag-mount mobile cell antenna apart and use the whip and it's own mount and attach it opposite the Monitenna. To the best of my knowledge, and since I'll have lost the ground plane (vehicle), I'll have to devise another one. What do I need to do? I'm assuming that I'll need a steel or aluminum plate to attach the whip to, an insulator of some type between the whip and plate, and then add some radials. How long do the radials have to be? How many are necessary? Is there a formula for these calculations? Am I wasting my time? I always put a dollar figure on everything I do. If I end up with a million hours in this project it would be cheaper in the long run just to purchase exactly what I need and be done with it!

Well... that's quite a lot of words for a newbie!!! I hope that I didn't overstep my bounds for a first post. I'd certainly appreciate any and all help, suggestions, or opinions.

Thanks for your time,
Monarch

 

[SCPD]

Not too sure what a Monitenna is, or what it looks like, so I can't offer any advice for that. If you want to mount a magnetic antenna for 800mhz you could mount some sort of metal plate on the mast. The plate wouldn't have to be too big for that frequency range and the magnet could stick to it. Waaaay back in the past, when "portable" cell phones still had a full size handset, I used a mobile magnetic antenna attached to an air conditioning unit inside a room with no problems. I've also used a cookie sheet to mount a quarter wave VHF antenna and it worked great. The cookie sheet provided enough counterpoise to give an adequate SWR. I was able to hear. and transmit to, repeaters from 25 miles away with the antenna inside the house.
If your mast is going to be taller than the length of the coax I would dismiss the idea altogether. The coax that comes with magnetic antennas are okay for mobile use but the standard RG58 is not suitable for lengthy runs at those frequencies. Several of the coax manufacturers publish a performance chart of their products. Look for the lowest dB loss per foot whenever possible. You'll notice that RG58 has quite a bit of dB loss at the higher frequencies like 800mhz. Something like Beldin 9913, or similar, will be better for long coax runs and use "N" connectors if possible. Times Microwave and Beldin make really good coax and stay away from the Radio Shack stuff if possible.
If you're going to monitor several bands at the same time, i.e., VHf, UHF, 800, a good discone antenna is good. Diamond makes a very good one. The only drawback to an antenna like this is when they're broadbanded they tend to lose gain. That's why I try to use better coax to help with the loss in gain from the antenna. In a perfect world we could have a dedicated antenna for each band to monitor!

 

[77monarch77]

Thanks for the quick reply frootydawg. Truth be told, I completely forgot about one of the most important things in a "base" set-up... the coax! If I trashed the magnet, would I need to use radials?

Well... let's see... I think I read somewhere that the first design ScanTenna was an inexpensive and poorly made knock-off of the Monitenna (originally but no longer made by ChannelMaster). It was identical except for quality issues. The second design ScanTenna has been modified and upgraded in many areas and the quality has improved vastly over the first design. So, I guess that it would be fair to say that the Monitenna and ScanTenna are, for all intents and purposes, clones.

Of course, all of the above information hinges on the fact that my memory hasn't failed or that I haven't been fed a bunch of B.S. by an unknowing website author. Either of which could be true. If so, I'd appreciate knowing the actual truth.

Anyone...?

 

[SCPD]

I would say "Yes" in that you would need radials if you trashed the magnet. Sometimes the magnet is an integral part f the antenna design and may actually perform worse if you trashed it. If you live in an apartment, condo, etc., it may be beneficial just to keep the antenna the way it is and attach it to a large piece of metal.

My guess is that most of the magnetic mount cell antennas are quarter wave resonant and they require some sort of counterpoise. When they're attached to the vehicle, are some other sizeable piece of metal, it provides the counterpoise for it. Otherwise, you'll need to fashion some sort of radial's to it for it to work properly. Try using it while it's attached to your refrigerator, air conditioner, etc., and then remove it. You'll notice a difference in the signal strength. Ever thought of building an antenna for 800mhz? It would probably be easier than modifying a magnetic antenna.

If the Monitenna is what I'm thinking it is it's a center fed vertical dipole antenna that extends out on an arm 90 degrees from the support tower.

 

[77monarch77]

Yes... I have thought about building my own 800Mhz antenna. I just thought (there I go thinking again) that it would be easier to start with the cell whip. Why... do you have an idea in mind or a direction that you can steer me towards?

And yes... you are exactly correct about the Monitenna description. Since I will be setting this up, probably, within a weeks time, I'm more concerned with the original aiming/bracing issues as per my original post. What do you think of my gusset/brace idea? Do you think that this is a valid concern? I had intended to attach a 3/8 steel rod (appropriately bent) in a triangle type fashion (45 degree angle to both the mast and arm) and secure each end with a worm clamp. I don't yet have the antenna "in hand" so I can't tell you it's weight.

Your opinion?

Regards,
Rob

By the way, I own the house and there are no neighborhood or town restrictions. The only people that I need to satisfy are myself, and of course, the little woman! So as long as my setup is not too obtrusive or ugly... and it works... I'll be all set.

 

[LarrySC]

The Monitenna is a 5094-A Channel Master which is no longer in production. The way you describe it I get the idea that you only have 1/2 of the antenna. It is 10' overall. 5' above the arm and 5' below the arm. All of the antenna is [+] and receives without any ground except the mast. It is a 300ohm ant and requires RG-6 75ohm coax. There is a 4:1 balun transformer between the coax and antenna. Check this unit with an ohm meter for any of the three connections for a reading. All should show 100% short. If one leg is open it may have received a lighting charge. I use one for my shop, one for my in home office. and one for my office 21 miles south. Note, however, that the total capture area is appx 275 total inches. This means it may be too much antenna in a metro area for some scanners. In my office at home I use a Radio Shack 15-578 attenuator to cut down on some of the incoming signal. Dont waste time building an 800 antenna when you cant get them ready to install. See www.bncantenna.com and look for 2-2067 with base adp. Good Luck, Larry

 

[77monarch77]

Hi Larry,

Thanks for the reply. You've given me quite a lot of info on this antenna. I appreciate it!

No... I DO have the whole thing. I don't know where in my posts that I screwed-up that would make anyone think that I only have half of this unit. Oh well...

I will check the balun as you recommended. I also live in a suburban-rural area but will keep an eye on reception for possible attenuator/filter usage.

My mind is not quite made-up as far as an 800Mhz antenna is concerned. Although, because I'm far from experienced when it comes to many of the aspects of antennas and tuning, I may have to (out of ignorance?) purchase one.

Again, thanks for your input.

Rob

 

[77monarch77]

I'm going to mount this antenna this weekend. I was reading somewhere else in RR about someone who had a ScanTenna that [he stated] seemed to be somewhat directional even though it's an omni-directional antenna. His solution was to install a second one on the same mast. I doubt that I'm going to do something like that, but I'm still curious about this antenna being "somewhat directional" and wonder if I should aim it in the general direction of my most frequently monitored frequencies. If so, which end would be the pointer... the arm end or the antenna end? If you look at a picture of a ScanTenna and took out the large dipole it would look just like a yagi. In that event, the arm end would be the pointer. But because the large dipole is most of the antenna, it can't be removed from the equation. So ????????