Looking for input on antenna placement options...

[kd8mkg]

So my question deals with coax losses, obstructions, and antenna height. This is for VHF and UHF civilian and milair.

In your opinion, would it be better to have an unbroken 45' run of LMR-400 to an antenna in the attic of a 1-story home, with the center of the antenna about 13 feet above ground level.

Or, would it be better to have 70 feet of LMR-400 with about a 2 foot jumper of RG-8X in the middle, with the antenna outside and the center of the antenna about 19 feet above ground level.

I'll be using N-type connectors for all cables.

I've done the coax loss math at several frequencies. At 400MHz, the the line losses are 2.1dB for the outside run, and 1.353dB for the attic run. So the attic run is the clear winner purely in terms of coax losses, but my attic is full of electrical wiring and other obstructions, and being in free air has to have some gain associated, as does the additional 6' of height.

My gut tells me height is might, and though 6' isn't a lot, it's kind of a lot in terms of radio horizon.

Thoughts?

Thanks!

 

[mmckenna]

Height is might.

I'd do the extra cable to get it up a bit higher.
It would also get the antenna away from RF Noise makers inside the home.
Not all building materials are RF opaque, so simply getting it outside and in the clear may be an improvement.

Coax loss is a reality, but you have some good trade offs that make the additional cable a worthy consideration.

 

[jtwalker]

Why 2’ of rg8 in the middle? Can’t you make one of the other pieces 2’ longer?

 

[rk911]

frankly, i think the difference between the options is so small as to make the choice irrelevant. there are things involved that are beyond your control. the location of the transmit sites, obstacles between your monitoring station and the transmit point(s) to name one. i do agree that a single unbroken run of coax be it LMR400, RG8x (my personal compromise cable), RG6, RG59, RG8...whatever. the fewer connectors between the back of the set and the antenna the better.

 

[kd8mkg]

I'm going through the weather-sealing under a door. RG-8X fits, 400 definitely does not. This can't be a permanent install (I eventually plan to erect a tower, but it will be at a different location). I don't want to drill a 5/8" hole through masonry for a temporary install.

 

[kd8mkg]

frankly, i think the difference between the options is so small as to make the choice irrelevant. there are things involved that are beyond your control. the location of the transmit sites, obstacles between your monitoring station and the transmit point(s) to name one. i do agree that a single unbroken run of coax be it LMR400, RG8x (my personal compromise cable), RG6, RG59, RG8...whatever. the fewer connectors between the back of the set and the antenna the better.

I'm right on the edge of my local airport's approach. I can hear it currently (using a 2m/440 j-pole in the attic), but it's only a few dB above the noise floor.... I'm right on the edge. I think the combination of an antenna tuned to the frequency (DPD Omni), removal of the RF obstructions present in the attic, and a few extra feet of height might be just enough.

Also, I'm up on a hill, 18 miles from the closest Indy Center RCAG site. For some reason, I cannot hear it, although I definitely should be able to, at least according to a radio coverage map generator I use. My current antenna might be in a null. Moving it outside to a mast would give me some wiggle room to see if I can bring that site in.

 

[kd8mkg]

I figured out why local approach/departure and tower are so weak (they should be quite strong at ~17 miles line of sight), while the other airport frequencies are booming in. I did an analysis of the RTL-SDR FM Notch that I have using my VNA, and it's down about -14dB at 118MHz!!! That means that only about 3% of the signal my antenna is hearing is getting into the receiver. Jeez, no wonder I can barely hear it. I ordered a Mini Circuits ZBSF-95+ to use instead, which claims -1.1dB at 118MHz.

My guess is that with a better filter, I might not need to put the antenna outside.

 

[Ubbe]

I did an analysis of the RTL-SDR FM Notch that I have using my VNA, and it's down about -14dB at 118MHz!!!

FM bandstop filter comparison

I rolled some numbers on the RTL-SDR FM bandstop filter and the HPN-30118 AM/FM filter using a VNA. The results are interesting compared to the ZBSF-95+, especially when you hit VHF Air voice at 118 - 137 MHz. The ZBSF-95+ numbers come from their spec PDF as I do not own that yet. There are...

forums.radioreference.com

You can look at the comparison of filters in the forum. Normally the band filter in a scanner attenuates FM broadcast, maybe 20dB, so you often do not need to attenuate 40dB in an external filter, that's 10.000 times. If you get the HPN30118 from scannermaster it will also cut away the AM broadcast band and will cost half of what MiniCircuit charge for their filter. I need to use the HPN one as I have lots of 2-way transmitters in the 66-87MHz band

/Ubbe

 

[kd8mkg]

FM bandstop filter comparison

I rolled some numbers on the RTL-SDR FM bandstop filter and the HPN-30118 AM/FM filter using a VNA. The results are interesting compared to the ZBSF-95+, especially when you hit VHF Air voice at 118 - 137 MHz. The ZBSF-95+ numbers come from their spec PDF as I do not own that yet. There are...

forums.radioreference.com

You can look at the comparison of filters in the forum. Normally the band filter in a scanner attenuates FM broadcast, maybe 20dB, so you often do not need to attenuate 40dB in an external filter, that's 10.000 times. If you get the HPN30118 from scannermaster it will also cut away the AM broadcast band and will cost half of what MiniCircuit charge for their filter. I need to use the HPN one as I have lots of 2-way transmitters in the 66-87MHz band

/Ubbe

Yeah, I would assume that to be the case, but also, I looked it up last night, and I'm within 6 miles of 6 different FM broadcast transmit towers, a few of which are 50kW. One of those 50kW transmitters is right at the top of the band. Without the additional FM broadcast band attenuation, the 15X picks up the AM carrier on Airband, but just spits out static. Completely unreadable. Put the RTL-SDR in front, cleans it right up, but the transmissions near the bottom of the band are weak, which is definitely due to that filter still attenuating well into the airband.

I considered the HPN-30118, but based my choice on this thread. Given my proximity to so many transmitters, the additional attenuation of the ZBSF-95+ is probably worth the additional cost. The additional loss at 118.000 (which is the primary approach frequency of interest) of the ZBSF-95+ compared to the 30118 is negligible, and since I'm already using LMR-400 with N-type connectors, that was another selling point for the ZBSF-95+. No need to buy additional tools or fuss with shoddy adapters. The AM band doesn't seem to cause me any issues. Most of the local AM transmitters are pretty distant from me.

When the filter arrives, I will, of course, characterize it for myself with my VNA to ensure that it is doing what's on the tin.

 

[popnokick]

Since you're concerned with the total dB loss in the feedline.... that 2 foot section of RG8 in the middle of the LMR-400 introduces the losses associated with the additional coax connectors you'll need on both ends of the RG8 and new connectors where you cut the LMR400. And also the two barrel connectors used to insert the piece of RG8. Have you considered a good quality RG6 for the entire run? RG6 is simpler to install and BONUS: RG6 will fit under the door. Connector losses would be gone and you could easily figure the loss for the entire run of cable minus the additional connectors necessary for the RG8.

 

[kd8mkg]

Since you're concerned with the total dB loss in the feedline.... that 2 foot section of RG8 in the middle of the LMR-400 introduces the losses associated with the additional coax connectors you'll need on both ends of the RG8 and new connectors where you cut the LMR400. And also the two barrel connectors used to insert the piece of RG8. Have you considered a good quality RG6 for the entire run? RG6 is simpler to install and BONUS: RG6 will fit under the door. Connector losses would be gone and you could easily figure the loss for the entire run of cable minus the additional connectors necessary for the RG8.

Yeah, I can and have calculated the system losses at various frequencies, not just the coax... so the numbers I'm using at represent all connections, filters and lengths of coax.

As an example, at 400MHz, the options are as follows:

For the
Antenna RG8x -> N ->LMR-400 25' -> N -> LMR240 2' -> N -> LMR-400 40'-> N -> Mini Circuits ZBSF-95+ -> N -> RG8x 1' -> BNC -> Radio
option.... system losses at 400MHz is 3.408dB.

On the other hand, if I don't use LMR-400 between the under-door section and the antenna, and just go from the indoor LMR-400 to LMR240 all the way to the antenna, and eliminate one connection point, the losses at 400MHz are 3.867dB. The benefits of eliminating a connection point are outweighed by the benefit of much lower loss coax, but only marginally. That difference will be negligible.... about 8%

The attic option is much lower loss, a total system loss of 2.526dB at 400MHz. And, of course, all of these numbers are much lower at 137MHz. It may not seem like much, but 3dB represents a 50% loss of signal strength. It can easily be the difference between readable and unreadable.

There are many reasons not to use 75Ohm coax in my case, but namely, I'd have impedance mismatches at each connection point (dependent on frequency, of course). RG6 would detune the system; not by much, but enough.

But most importantly, I already have all of the rather expensive tools for LMR-400, LMR-240, and RG-8X, and also, a bunch of leftover N connectors for these cable types. I don't have RG6 tools or connectors. That nickel and dime stuff adds up!

All of that being said, I do have a much better picture now, than I did when I originally posted this thread. My problem was the RTL-SDR filter... I thought I was at the edge of coverage, but it turns out, that filter is just attenuating signals at the bottom of the band, namely 118.000 and 118.850, the approach/departure frequencies, and 119.900 (Tower) by about 12-14dB!!! Only ~3% of the signal is reaching the receiver. Once I receive the ZBSF-95+, which has much better performance figures near the bottom of the band, app/dep and tower should be booming into my radio.

This makes sense... I'm located in Dayton, about 17 miles from the airport, and Dayton is in a valley. I'm on the Southern hill coming out of that valley... I basically have line-of-sight to the airport from my roof.

I suspect I will have no need to put the antenna outside, and also, I need a ~40' length of LMR-400 for either outside or attic... so I'm going to try the attic first. My guess is that I will have excellent coverage of Dayton International, Wright-Patt and the local Indy Center RCAGs with the DPD Omni and a better suited filter. I will post an update when I have everything setup.

 

[Ubbe]

Antenna RG8x -> N ->LMR-400 25' -> N -> LMR240 2' -> N -> LMR-400 40'-> N -> Mini Circuits ZBSF-95+ -> N -> RG8x 1' -> BNC -> Radio option.... system losses at 400MHz is 3.408dB.

Maybe install that FM filter and a preamplifier at the antenna and be gone with all worries about coax losses and impedance matches. You could also switch to the more manageable RG6 which can use twist-on connecters at $2 a pair and needs no tools, except a $5 coax stripper, if it would be more suitable and less error prone without all adaptors.

There will always be a big loss when having an antenna indoors in the attic compared to outdoors, especially if it can be installed higher up, but it will be more protected indoors.

As you have that VNA, measure your antennas over the working frequencies and see what impedances you see. Do the same with your scanner when on hold on a frequency.

An amplifier at the antenna and a splitter at the scanner will isolate any impedance mismatches and keep the impedance constant to antenna, coax, and scanner so that one of them do not ruin it for the others.

If you use a standard CATV $5 splitter you can often pry the lid off and cut the trace that goes to the input connector and solder a small 2pF capacitor to bridge the gap, and add a 86 ohm resistor from the connector to ground to load the coax properly. It will create equalizing so that higher frequencies have less attenuation, to compensate for the coax that works in the opposite way. Electronics 2000 | Reactance Calculator You will also need additional attenuation and best are a variable 0-20dB to be able to dial in the perfect amount of attenuation.

/Ubbe

 

[kd8mkg]

Maybe install that FM filter and a preamplifier at the antenna and be gone with all worries about coax losses and impedance matches. You could also switch to the more manageable RG6 which can use twist-on connecters at $2 a pair and needs no tools, except a $5 coax stripper, if it would be more suitable and less error prone without all adaptors.

There will always be a big loss when having an antenna indoors in the attic compared to outdoors, especially if it can be installed higher up, but it will be more protected indoors.

As you have that VNA, measure your antennas over the working frequencies and see what impedances you see. Do the same with your scanner when on hold on a frequency.

An amplifier at the antenna and a splitter at the scanner will isolate any impedance mismatches and keep the impedance constant to antenna, coax, and scanner so that one of them do not ruin it for the others.

If you use a standard CATV $5 splitter you can often pry the lid off and cut the trace that goes to the input connector and solder a small 2pF capacitor to bridge the gap, and add a 86 ohm resistor from the connector to ground to load the coax properly. It will create equalizing so that higher frequencies have less attenuation, to compensate for the coax that works in the opposite way. Electronics 2000 | Reactance Calculator You will also need additional attenuation and best are a variable 0-20dB to be able to dial in the perfect amount of attenuation.

/Ubbe

I'm certain your solution would work, but it's a bit like adding a supplemental pump to a small water pipe in order to increase pressure and velocity to make up for inadequate size. If I had moved into a home with a tower and some RG6 already running up the tower, sure I could do that, but since this is a new install, why not just size the pipe appropriately?

Also, one of the approach frequencies at my local airport is 134.450. This frequency is booming in, full quieting, RSSI reported by the 15x at around 850+. 134MHz is above the roll-off of my current filter, so it likely predicts the signal strength I can expect at ~118MHz once my upgraded filter arrives. If I put a preamp on that with 18dB of gain or whatever, it might overload the front-end.

Luckily, I will be able to do this in discrete steps:

1. My new filter will arrive first. I am currently using an existing run of LMR-400 that is a straight shot to an Arrow 144/440 j-pole in the attic. I will replace the RTL-SDR filter with the ZBSF-95+ filter, this should give me a decent picture of coverage.

2. My new DPD Omni antenna will arrive next. I'm going to stick it in the attic, unhook the existing LMR-400 from the j-pole and run it over to the Omni. This should give me a final answer as to whether attic mounting will be adequate for full coverage. I expect it will be, but don't know until you try.

3. Assuming that all goes well, I'll do a second drop of LMR-400 into my shack. Done and done. For the time being, I'm just going to unplug the antenna from the scanner whenever I hop on 2 meters or 70 centimeters, which is pretty infrequently. I'll eventually install something like a LIM-01WB at the antenna to protect from my transmission (if you are wondering, there will be about 20 feet of separation between the two antennas, and I rarely transmit with more than 5-10 watts)

4. If for some reason the attic mount doesn't give me adequate coverage, I'll cross that bridge when I come to it.

I don't mind working with LMR-400. Sure it's a bit heavy and cumbersome, but not unmanageable.

 

[prcguy]

In almost all cases taking on a reasonable amount of feedline loss to get an antenna up in the clear and away from obstacles will improve reception. I would put the antenna outside.

 

[kd8mkg]

In almost all cases taking on a reasonable amount of feedline loss to get an antenna up in the clear and away from obstacles will improve reception. I would put the antenna outside.

I certainly don't disagree, but that comes with so many pain points. How to get the coax outside, It would essentially be a mast attached to one of the beams of my deck, so I'd have to go raise it to listen, lower it when done. Also, there are 240V power lines above the other side of the deck, so I'd have to worry about that. Wind, ESD, lightning, all the things.

Like I said in a previous post, I plan to eventually erect a tower, at which point I can deal with all of those things, for the time being, I can already hear Indy Center (though a bit scratchy), and all of my local airports Approach, Departure, Tower and Ground. Ground is down in the weeds, but it's also on a frequency that's being attenuated by that RTL-SDR filter by around -12dB. I bet I'll be able to hear it much better with the Mini Circuits filter.

Moving it outside is just a matter of adding more, won't need to replace anything.... so I can always do that if I hit a point when I'd like more signal.

 

[Ubbe]

If I put a preamp on that with 18dB of gain or whatever, it might overload the front-end.

"You will also need additional attenuation and best are a variable 0-20dB to be able to dial in the perfect amount of attenuation."

I don't mind working with LMR-400. Sure it's a bit heavy and cumbersome, but not unmanageable.

As you already have connectors and tools, the expensive part of it, it's probably fine but you had that narrow space that needed RG8 so I though an unbroken length of RG6 would be a better choice. An advantage with amplifier are that it has a noise figure of less than 1dB and the scanner has 5-6dB. Having the total gain from antenna to scanner at 6dB will improve sensitivity with about the same 6dB. But you seem to have all the signals that you want to monitor at full strength, except that 118MHz frequency that probably will be fixed by a more suitable filter. But if you later need to receive a system that's too weak you'll now know how to improve weak signals.

/Ubbe