Upgrading from OmniX to something more directional (VHF)

[hfxChris]

Right now I have the DPD OmniX antenna which, I have to say, is a great omnidirectional antenna and I'm quite happy with it. However there are a couple of VHF repeaters (~150-155 MHz) used by fire departments in neighbouring counties that I can receive now, but they're fairly noisy. They're all located in the same general direction from my home, so I was thinking something that might give me some additional gain in that direction might be worth considering.

Anyone have any recommendations? At my current location unfortunately I'm limited to discrete antennas, the OmniX for example is in my attic. I suppose I could try moving it outside as well temporarily. I'm also not against the idea of building something myself, but there seem to be a number of different yagi calculators for example, but they all give somewhat different results.

Any advice is appreciated!

 

[vagrant]

You may need a different or additional antenna, or you may not. Tell us about the coax type, length and any adapters on the line from the OmniX to your receiver. Additionally, how far away are these other repeaters? At least a guess will help with analysis toward the current/future antenna system.

Also, does your receiver have a signal strength meter? If so, do those repeaters have a strong signal, but noise, or weak signal as well. A variable gain pre-amp may help, but if the "noise" is due to interfering signals...not so much without filtering. (Interference may be the sole cause. For example, FM broadcast filters help me, but your location is not mine.)

 

[mmckenna]

Right now I have the DPD OmniX antenna which, I have to say, is a great omnidirectional antenna and I'm quite happy with it. However there are a couple of VHF repeaters (~150-155 MHz) used by fire departments in neighbouring counties that I can receive now, but they're fairly noisy. They're all located in the same general direction from my home, so I was thinking something that might give me some additional gain in that direction might be worth considering.

Anyone have any recommendations? At my current location unfortunately I'm limited to discrete antennas, the OmniX for example is in my attic. I suppose I could try moving it outside as well temporarily. I'm also not against the idea of building something myself, but there seem to be a number of different yagi calculators for example, but they all give somewhat different results.

Any advice is appreciated!

Putting an antenna in the attic can have some "gotcha's". It's closer to noise sources in the home. Any wiring, metal ductwork, metal plumbing, etc. in the attic space and be an issue. If there is foil backed vapor barrier/insulation between your antenna and the outside, that can be a problem.

If you can, take your scanner up on the roof and give it a try with the stock antenna. That can tell you a lot.

If VHF is the area of interest and all stations are in the same general direction, a simple 3 element VHF Yagi can help. You can make these fairly easily using plans online, or you can buy them.

But I agree with Vagrant, often issues lie elsewhere, tell us about your current setup before dropping your money on a new antenna.

 

[Ubbe]

A cost effective solution could be to buy a cheap tv antenna for band III 175-230MHz that have 5-6 elements and increase all the distances between elements by 30%. The boom lenght will not be enough for all elements so you'll have to reduce the design by one element at the front.

Cheap antennas usually have simple attachments of the elements, a hole in the boom where the element are snapped in place by some plastic holder and it's just a matter of drilling new holes. The element at the back are used as the starting point and then usually comes the active dipole that needs to be moved 30% forward on the boom. The old distance between dipole and the directory element in front of it needs to be increased by 30% so that new distance need to be measured from the new posisition of the dipole. Continue doing that with all elements.

Start by measureing the antennas original distances between elements and write down the values when they are increased by 30%.
The more elements the antenna has the more directivity and narrow beam it has but also more gain. The lenght of each element are frequency dependent but lenghten all elements are more complicated and expensive and the crucial part of a yagi antenna are the wavelenght between the elements. The modified antenna should work well between 120Mhz and 160Mhz with its most gain at the highest frequency.

/Ubbe

 

[KMG54]

Ubbe forgot, the antenna needs to be mounted vertically Look up Stellar labs VHF TV antennas..

 

[hfxChris]

Thanks for the responses!

To answer the questions, the coax is approx. 30 feet of RG59 (Belden 8241). Currently the only adapter in line is a BNC to PL259. The receiver is a Kenwood TK-7180, which uses an SO239. I also have a Par notch filter configured for 164 MHz to knock down a strong nearby DMR repeater I use with my Uniden scanners, but with the Kenwood I haven't found it's made much of a difference, so I'm currently not using the filter with it.

The repeaters are all around 50-60 km, 30-40 miles. The Kenwood does have an s meter, but I'm not sure how useful it is. It appears to have only 3 bars at the max, and these repeaters usually show 1-2 bars.

I'm hesitant to put anything in line that would amplify it, like I said above I have a very strong DMR repeater on 164 MHz less than 2 km from my house which completely swamps my Unidens. The Kenwood seems to handle it a lot better though.

Also for what it's worth, my house is on a hill, actually one of the highest locations in the immediate area. I've used the online elevation tools which show the elevation profile between two locations, and there's very little standing between myself and the repeaters. I'll probably try moving the OmniX outside for a bit as well, see how that compares.

Thanks again everyone!

 

[KMG54]

Get rid of the RG59, cheap but effective goodRG6 solid copper, easy to work with and BNC compression fitting are available.

 

[mmckenna]

Yeah, RG-59 is pretty well near the bottom of the barrel.

 

[pro92b]

RG-6 cable does have less loss than RG-59. Each situation should be analyzed to decide if the difference is worth considering. In this case the frequency of interest is 155 MHz and the cable is 30 feet long. Using the calculator at - Times Microwave The loss for RG-6 is 1.1dB and for RG-59 it is 1.4dB. The 0.3dB improvement from replacing RG-59 with RG-6 in this case would not result in a noticeable improvement.

 

[mmckenna]

RG-6 cable does have less loss than RG-59. Each situation should be analyzed to decide if the difference is worth considering. In this case the frequency of interest is 155 MHz and the cable is 30 feet long. Using the calculator at - Times Microwave The loss for RG-6 is 1.1dB and for RG-59 it is 1.4dB. The 0.3dB improvement from replacing RG-59 with RG-6 in this case would not result in a noticeable improvement.

Good point. And I agree, moving from RG-59 to RG-6 likely isn't going to make a noticeable difference. But there are different grades of each cable. The online calculators usually only offer up one or two specific brands.

But what sort of state is the cable in? Were the connectors installed correctly?

OP says he can hear them on the 7180 but signal level is low. .3 dB probably isn't going to make enough of a difference.
But couple in a better antenna with better coax and doing away with adapters, getting the antenna outside, and that can easily improve quite a bit.

 

[vagrant]

1. Sometimes moving an antenna one meter in any direction can help. (Horizontal movement)

2. There is a cost to purchase RG6 and it would be an improvement. Still, the cost is quite low.

* 3. Not knowing exactly where you are, or the repeaters in relation to your position, I can tell you that the 104.3 MHz FM radio station CFRQ in your locality reportedly has 100k watts ERP which is significant. That and other FM high power radio stations can suppress signals on other frequencies as well as the specific freqs around 150 MHz that you want to monitor. An inexpensive FM filter may help. They definitely help me and it will probably help you. You may need some adapters to put that filter inline for your receivers. Do not TX with that FM filter inline, but you probably know that.

4. While a yagi would help, it could also increase the RFI you are more than likely receiving from that FM station and others nearby.

I personally use FM & AM filters for receivers at home, mobile and handheld. While more expensive receivers can have circuitry to better handle RFI, the power broadcast stations use is significant and can usually be addressed with a filter. Try an FM filter first and see if that resolves the issue.