Switched antennas need insight

[nick1427d]

Background of what I'm attempting to monitor:

I have 2 BC780s that I use to only monitor fire frequencies in the 150-160 ranges.
1 BC780 I use for dispatch channels was connected to an external VHF antenna 3db gain(antenex B1443) nmo mount trunk bracket attached to my house gutter on 2nd story of house. Run is approx 20ft of RG58 terminated to BNC.
Even though I know this setup is not ideal, it worked for me cause I didn't want to put anything up permanently.

The other scanner is for response frequencies and is connected to a mag mount comet dual band with 15' RG58. I have no issues whatsoever with this setup.

Here's the issue:

The B1443 was a bit too noticeable for me, being taller than a unity gain. Today I put a VHF unity gain in it's place which obviously is about half the length. I noticed significant changes in my stations. Most noticeably being more "noise". I'm not sure if this is overloading or intermod. I'm 20 miles from heart of St. Louis and about 3 miles from the city across river in IL. I messed with attenuation which made it worse, changing modes to NFM from FM didn't help. Note that from my research I thought that a unity gain vs a 3db would not matter because I have no ground plane. Note that this interference is not noticeable on all the frequencies I'm monitoring.

So the question(s):

Does this sound like intermod and/or overloading due to improved efficiency of the different antenna?
Could I be getting interference from an outside source? Recommend a filter?
Should I put some ground plane radials on this bracket and see if it improves?
Could the aluminum guttering be causing the interference?

Obviously the B1443 made for better audio but I have not found a decrease in my distance reception since changing the antenna, which is my goal. I know that height would be my best improvement but I'm wondering if making a ground plane would work? Also note that in the immediate area of this antenna, which is at the end of the guttering only has the obstruction of the house on a small area and no trees or other buildings.

 

[N5TWB]

So the question(s):

Does this sound like intermod and/or overloading due to improved efficiency of the different antenna?
Could I be getting interference from an outside source? Recommend a filter?
Should I put some ground plane radials on this bracket and see if it improves?
Could the aluminum guttering be causing the interference?

Obviously the B1443 made for better audio but I have not found a decrease in my distance reception since changing the antenna, which is my goal. I know that height would be my best improvement but I'm wondering if making a ground plane would work? Also note that in the immediate area of this antenna, which is at the end of the guttering only has the obstruction of the house on a small area and no trees or other buildings.

Q#1 - Not likely. If anything, it would have been the other way around.

Q#2 - Possible but the other antenna should have also been affected so I don't see a filter being part of the solution.

Q#3 & 4 - If I understand your install, the gutter is acting as a ground plane with the bracket being attached to it so it is not a source of interference because the B1443 would have also been affected.

Here's a link with antenna model charts to show you the difference in the quarterwave and 5/8 wave antennas:

5/8th wave mobile antenna vs 1/4 wave

The primary take-away is that the 5/8 wave antenna works better toward the horizon, thus increasing range. The quarterwave has more vertical characteristics in its pattern so it doesn't have the horizontal range. My opinion is that is the source of your problem if I'm properly understanding your install and symptoms.

 

[nick1427d]

So in your opinion the guttering is creating a ground plane? Do you think I'd benefit from attaching "grounding rods" to the bracket?

I'm thinking that the B1443 wasn't receiving a nearby paging tower due to hits horizon characteristics. The 1/4 wave being more vertical is probably picking it up. It's only 5 miles away.

 

[N5TWB]

So in your opinion the guttering is creating a ground plane? Do you think I'd benefit from attaching "grounding rods" to the bracket?

I'm thinking that the B1443 wasn't receiving a nearby paging tower due to hits horizon characteristics. The 1/4 wave being more vertical is probably picking it up. It's only 5 miles away.

As long as the raingutter is metal and electrically connected to the coaxial shield as you described your mounting system, it is part of the ground reference for the antenna. It might be an interesting experiment to add some other elements to create a 360-degree ground but the benefit might be minimal.

Paging towers are notorious for overloading the front-end of most scanners so your report of not receiving it is puzzling, no matter the antenna or its design. That's a mystery I can't solve from a distance.

 

[benbenrf]

Nick

This ground-plane/counterpoise added to the by way of the gutter is going to:

- re-shape the radiation pattern in terms of both azimuth and elevation - by just how much and in which a direction and at what angle is going to be a function of the gutter length and the pattern/shape established as it snakes around the house. What you can be dead cert about is that if you connect a vector analyzer and run s-parameter and other tests you are going to find your antenna no longer displays anywhere near its original pattern!

If the gutter system has metallic down pipes attached they are really going to make a circus performance out of your antenna, but by far the 2 most common overlooked problems associated with metal gutters/downpipes used as grounds or counterpoises for antenna systems come about through:

- a domestic electrical earth connected to them, usually by way of one of the down-pipes, somewhere at some point around the house. This can be a recipe for all kinds of “noise” getting into the system.

- galvanic corrosion - often less appreciated but potentially a worse problem regards “noise”. Any oxidation or rust between the anti-rust galvanization applied and the base [metal] to which it is fixed generates a voltage which will = “noise” – and big time noise it can be.

In short: using a metallic gutter system as a counterpoise or ground has the potential to **** things up real good, to put it bluntly. Ironicaly in HF and VLHF setups - with the inclusion of a decent low pass filter - very long and straight gutter layouts have the potential to be excellent antenna systems.

 

[nick1427d]

Wow my head hurts. LOL thanks for the very technical insight. I should've noted more that I'm only receiving and no transmitting. The vhf quarter wave is attached to the guttering via a simple trunk mount. It was no intention of mine to use it as any form of "ground".

Should I look into a filter?

 

[benbenrf]

I'm a great believer in filters - in particular cavity type band pass or pass band filters. They cut out everything - above and below the frequency you want to listen to. Problem is, they are not cheap if purchased, but they can be DIY'd at home with some background comprehension of the theory and a basic mechanical type workshop with access to metal working.

The big advantage with good filters (and I mean good as opposed to cheap inline coaxial ceramic types is that they remove from the antenna/coaxial input to your receiver all the junk and noise that your receiver would otherwise have to deal with before it can extract the signal information it needs to demodulate to rf and provide you with audio output. Now, lets get things in context: your typical receiver/scanner is going to work without any additional filtering in most cases, but until one actually uses a good cavity type filter in a noisy rf enviroment, you don't realise just what a huge difference they can make to received signal quality.

All filters introduce signal strength loss, that's an unavoidable characteristic of filters, but loss of signal strength is less the problem than most folk realise. A far bigger problem is background noise - especially in and around urban enviroments. More often than I think many folk realise, poor reception is not because they have a weak signal o start with, but because their scanner/receiver simply cannot deal with the noise in and around the signal of interest you want it to tune into and demodulate. This is Signal to Noise Ratio, or SNR - in other words no matter how strong or weak the desired signal is unless your receiver can recognise it from any noise around it, it simply is not going to "pull it out" and seperate it from the interferance.

So why is this a problem with off-the-shelf consumer type receivers and scanners?
Good filters cost money - they are not cheap to manufacturer and introducing them into consumer type receivers stuffs up the budget. They also take up volume and space in proportion to the frequency they are designed to work on - that means the scanner/receiver manufacturer has to increase the size of the chassis/box. So a comprimise is made with off-the-shelf type scanners/receivers - they are sold with cheap filter designs, an evil neccessity to be able to put the product on the market at a competative price, and if the end-user wishes to invest in a good quality after market filter, then they are free to do so.

The mistake a lot of people make when they have poor signal quality is that they think they need a pre-amp to boost the signal. That may be true in some cases, but my experience is that in 80% plus cases where folk thought a preamp added would improve things, it turns out they make things worse.

Why/How? - because preamps not only increase the signal strength level, they also amplify the noise by the exact same amount, now presenting the receiver/scanner with even more noise than it was having to deal with at the start. They do nothing to improve the SNR, when that is exactly what is required i.e. a solution thatincreases the desired signal level while not increasing the surrounding noise level.

So - is a filter going to solve your problem? On balance my opinion is yes. Let me requalify my statement: you have stated that you can hear audio - correct? This means that you have sufficient signal strength so a preamp is not required.What you can do is go online to eBay and search for bandpass filters: go through the list of all the bandpass filters that show up - and there could be hundreds of them - and search for those that band pass that frequency you want to listen on to. You want to find a band pass filter that is going to pass frequencies that fall between around 145Mhz - 165Mhz - something along those lines, or however high up in the 160Mhz plus range you need to cover. You need a pass-band filter with a centre frequency of about 155Mhz with a bandwidth of roughly 20Mhz, or roughly +/-10Mhz each side of 155Mhz - and that is more or less how the bandpass filter is going to be described by the seller: the center frequency will be stated first, and followed by a bandwidth or bandpass of +/- x Mhz each side of the center freq. This +/- whatever bandpass range will often, but not always, be quoted with the figure "3dB" attached. this means any signle that falls within the given +/- bandwidth will not be attenuated by more than 3dB or 50% of its original strength. That's how filters work: the closer you get to the edge of the given/allowable bandwidth the stronger and faster permitted signals start to be attenuated. Any signal on a frequency that is not within the +/- bandwidth is going to be attenuated big time. If you get a chance to look at a bandpass filter graph you will see that signals on frequencies outside the +/- bandwidth drop off strength quicker and quicker as they move away from the permitted bandwidth, and very quickly they are attentuated so much that they are so weak the receiver just hasn't got the sensitivity to pick them up.

Okay ... nuff' said - thats my 2cents worth for the moment. Go online eBay, take a look at band pass filters that fit your criteria, choose half a dozen or so that fall within whatever you consider an acceptable budget over the next day or so and send me their eBay item numbers by private message. I'll look at them on line and come back to you with my opinion about each of them (i.e. their pro's and con's etc etc .....) within 6hrs - 12hrs. Don't worry about spending a load of money on a real expensive good quality type - the object of the exercise here should really be just to make the point and demonstrate what a difference they can make. If you like the results, and if you then want to, you can always purchase a high quality one later.

Good Luck and happy filter hunting

Patrick.

 

[nick1427d]

Thanks Ben!

I should reiterate that the major problem is not a decreased reception distance, so I'm not even thinking preamp. Theres really only one frequency that is giving me an issue (154.325). On this frequency I noticed yesterday the scanner land on it and I got that garbled mess almost like a fax machine and then I could hear the NWS....from....162.550mhz. I could distinctly make out: "...we broadcast on a frequency of 162.550 mhz" blah blah

Could the NWS tower be that close to me? Is it just interference from something else?

I'm still debating on which antenna is working better for me. I've switched both unity and the B1443 and the only difference I notice is that one channel. On the B1443 I do get more bleed over it seems.

I'm gonna hunt down some filters and give it a try

 

[benbenrf]

...... is it just interference?

That's exactly what it is - but "interference" has many meanings and can mean many things. No I very much doubt this is a case of needing a pre-amp - I think that's been clear from the start. What you describe sounds like 2 rf products mixing to create a 3rd i.e. the desired signal mixing with another that could be partly or totaly within scanners' chosen IF bandwidth. If you hear something like this again very quickly try and change the associated bandwidth setting (will your scanner/receiver allow you to do this at the press of a button or flick of a switch independent of other settings?) and see now how the demodulated audio changes in quality and sound.

The above is an example of a signal that has challenged your receivers'/scanners' selectivity. Selectivity is the ability of the hardware to recognise the desired signal and process it (i.e. demodulate it) through to AF (audio frequency) in the presence of interference from other signals that are on/around the same frequency. This problem also rears its problimatic head when harmonics of the fundamental tuned frequency bleed through the front-end and are not filtered out.

What you are hearing could also be a example of saturation - a single signal or combination of signals that present your scanner/receiver front-end (usually the first IF and/or mixer stage) with an rf input that is strong enough to cause what is known in the trade as gain compression or front-end overload. It is closely related to selectivity. You can try adjust the receiver attentuation but things however are not as simple as they may first sound however, because gain compression or front-end overload can occur even when receiving signals that are well within, or well under the max reccommended or permitted input level quoted by the scanner/receiver manufacturer ..... but that's another story. So attenuation does not always correct overload or saturation - if you want I'll run through this theory in greater tech detail but I suspect you will find it quite boring.

So ultimately, yes this is "just interference" but to accurately identify the interference and what is causing it will require ideally a signal or spectrum analyzer. I still think a starting point here will be to choose a frequency and invest in a $20 - $30 band pass filter and use - just to see what a huge difference it can make - that or go back to the original antenna set-up because I doubt the gutter is helping things much!!

 

[nick1427d]

Thanks again Ben. I will be looking at filters today. I want to add again that I didn't mount the antenna thinking the guttering would effect it good or bad, I just need a place to mount my antenna outside and not drill holes.

 

[benbenrf]

Thanks again Ben. I will be looking at filters today. I want to add again that I didn't mount the antenna thinking the guttering would effect it good or bad, I just need a place to mount my antenna outside and not drill holes.

Understood ... loads of folk do that and in many cases they are happy with the results. All the theory aside, you know the old saying: if it works, what the hell..........