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Help with making a VHF low/high antenna

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fire42man

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So for the past few days I've been trying to figure out ways of building a VHF antenna, which I plan on connecting to a Bearcat 210 a friend is giving me. I will use it to scan some frequencies in the 46Mhz and 154-156Mhz ranges. So far I've built a 3-element yagi out of coat hangers and curtain rods. It does work, but the signal still isn't strong enough. The VHF low channels I'm tuning to are transmitting from about 30 miles away, and about the same on the high band. What I need is some help figuring out what kind and how big of an antenna to make. For a start, I have plenty of RG-59 cable.

I'm hoping for something that isn't too big, and not a yagi, because mounting either on the roof will annoy my neighbors (maybe I could tell them it's for HDTV) but, if it can fit in my attic that's where it will go.

Thanks for some help.
 

majoco

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A 46Mhz dipole will be about 11feet from tip to tip.

You can use old TV antennas for this job. There are plenty here that cover the old TV band 1, 2 and 3 frequencies and get the UHF too!
 

wmbio

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"I plan on connecting to a Bearcat 210 a friend is giving me" A nice gift.
You may not be the happest with the sensitivity of the older 210. The radio was built in Circa 1979-1982. With a wide front end. L&H band sensitivity averages .6Uv/12db SN. Many of the newer scanners even the lower cost units here in the classifieds are twice as sensitive and my help you hear better than building the perfect antenna.

Enjoy
Wmbio
 

LtDoc

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The one thing that would do the most good will be impossible if that antenna is in your attic. More height is the easiest way to hear more at VHF/UHF. So, any tall trees around your yard?
- 'Doc
 

fire42man

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There's some tall trees in my yard, but they're too far from the house and on the property line. I guess I could just mount the dipole on the back of the house. My bedroom window is above our patio roof, and from there I could attach the antenna to the main roof without the use of a latter.
 

W2PMX

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A dipole is 0 gain (it's the base measurement for gain). For a gain antenna you have to go with a Yagi, and the reflector will be a little over 11 feet long. The spacing between elements will be about 5 feet, so even for a small Yagi, 3 elements, you're talking about an antenna that's 11 feet wide and 10 feet long. You can't get gain and small size in the same antenna, and capture area also figures into how much signal you'll receive (the larger the area of the antenna, the more signal).
 

fire42man

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So after a trip to the hardware store to pick up PVC parts, I finished putting together a dipole. It is about 8 ft in height and I made it with two 4ft metal rods that were luckily laying around the house. I'm getting great clear signals so far, but I have to move it around to get the intended signals.
 

fire42man

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Sorry to bring up an old topic, but I am still having receiving issues and I discovered why. My neighborhood is behind a large hill, which of course is in the direction I'm trying to receive signals from. When I was scanning mobile once, I was able to get the good signals I intended with only an 18" whip. I was listening to a transmission when I got back in the neighborhood, and that's when the signal went dead. I know this is because of the hill. I built a 4-element half length yagi for the low band frequencies, but still had no luck. I don't know what I should do now, if I should get a preamp or not. Putting up a tower won't work due to the local regulations, and I'm stuck with using the attic. Should I build a new antenna or get a preamp?
 

mmckenna

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Low receiver sensitivity may be the common issue here. If you are not getting a signal with a 4 element beam, and you are using a reasonable feed line, there isn't much you can do but try a radio with better sensitivity. The older scanners were pretty weak when it came to low band reception. You might want to try a newer scanner, if you can find someone local that can loan you one.

You didn't really supply enough info to analyze the problem. Likely if your 4 element antenna is polarized correctly (vertically) and you have a decent coax run and you still are not getting anything, adding a preamp will be unlikely to help. Bigger antennas on low band are really going to be limited by your space. Usually land mobile radio systems are designed to work in a vehicle with a reasonably good antenna. On low band, about the best you can do on a car is a 1/4 wave whip. If your Yagi is constructed correctly, you already are at an advantage. If you still can't pick it up, I really doubt there is anything you can do short of moving the hill that is between you and the transmitter site.
 

mmckenna

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Oh, and unless you are really using some really bad coaxial cable, changing that likely won't help much. Feed line losses on low band are pretty low. Even going from something like RG59 to LMR-400 isn't going to help much.
A 50 foot run of RG59 at 46MHz is only going to loose about 1.5db of your signal.
Going to something "better" like LMR-400 is only going to drop your losses to about 1/2 a DB.
Going to ridiculous mode: 1/2" heliax is going to get you about a 1/4 of a DB loss.

You likely wouldn't notice the difference, especially if you are getting nothing at all now.
Here is the rule with preamps:
You can't get something for nothing. Preamps increase ALL the signals, not just the one you want. Likely adding a preamp will just increase the noise, again, if there isn't something useable there already, a preamp isn't going to help.
 

LtDoc

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One aspect that's probably contributing to not hearing what you want is the antenna's polarity, vertical or horizontal. If a signal isn't exactly 'strong', then the wrong polarity can certainly make it unhearable.
That hill between you and what you want to hear is probably the main culprit, especially at the higher frequencies (as you seem to have found out). Simplest solution is to 'move' that hill. Didn't say practical, just simple. Or the other side of that coin, you move your residence. Yeah, right. Oh well, sometimes it's just not possible to hear what you want unless you do something really 'drastic'. Sorry 'bout that.
- 'Doc
 

fire42man

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The yagi was horizontal, because I have no space to put it up vertical. I use RG-59 because that's all I have. We've barely lived at this house for a year, so moving won't be an option for a while.
 
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