FM Signal strength - When to take action?

[Flint Ridge]

Curios as to if I have enough FM filtering for my future antenna setup. Been working and getting ready for some upgrades before football season and before I get things all put together I was just wondering if one FM trap is enough for my location.

My primary station I have issue with is 34, which we have chatted about before and they went to half power along the way. I've finally gotten high enough to get to 2 Edge vs Tropo. Future setup will also have Kitz Tech - KT-500 amp with an FM filter built in.

http://www.tvfool.com/?option=com_wr...82302a495e1279

FM Fool Report
http://www.fmfool.com/index.php?opti...pper&Itemid=29

[ADTech]

It is necessary to post the image of the FM Fool report, that tool does not provide a means to linking to the report.

You'd also need to obtain specifics on the FM filter in the KT amp since there are no published data for it.

I've finally gotten high enough to get to 2 Edge vs Tropo.

Keep in mind at the TVFool modelling engine is ONLY an approximation and is based on data of limited resolution. There are inherent limitations in it that make the results of small changes in the input height pretty much an exercise in futility and that will probably make those results useful for entertainment purposes only. The real data comes when you actually put the antenna up in the air and probe at different levels to see what signal is actually there.

[Flint Ridge]

Sorry about the post mistake.

Here is the image of the FMFool, which has a fair number of LOS transmitters. I'm just not familiar enough with the signal strengths to know what might be too much.





Here is what the KitzTech KT-500 looks like on the inside - don't know if there is anything that looks familiar there. What I should have done was offer to drop ship it to you for "official" testing.

[ADTech]

Kitz would need to provide you with a graph of the FM filter's performance, a photo reveals nothing about the filter's parameters. Sure, I can test it if you'd like. Haven't seen a sample of that model.

I'd just throw a HLSJ (cap the L input with a terminator) on the front end of the pre-amp and be done with it since you don't have any low-VHF to worry about.

FM usually causes problems on high-VHF first, but an amp, in the presence of any strong signal, can go into compression and cause weak signals to be lost. It's simply better to keep the strong signals out of the amplifier in the first place.

[Flint Ridge]

ADTech, if I buy another one as a backup, I will touch base with you for testing of the KT-500.

The fog is kind of clearing now with the concept of the HLSJ to truly scrub out those signals. I'll have two separate antennas, one UHF and one VHF-Hi combined via UVSJ. So the actual sequence will be (UHF antenna - KT-500 #1) and (VHF-Hi antenna HLSJ ran "backwards" fed into the combined, cap with terminator on VHF-Lo, then VHF-Hi outputt into KT-500 #2). Coax #1 & Coax #2 combined properly with UVSJ for my viewing pleasure.

Guess I'm ordering an HLSJ and not putting up the VHF today, which is fine. Next weekend is a Holiday and if it does not rain then I might get everything done.

Thanks for the input!

[Flint Ridge]

Thinking - does the HLSJ pass power on both sides in either direction? Or is that like the UVSJ's where it depends upon the brand?

[ADTech]

Your sequence is wrong, plus you only need one amp.

Connect the two antennas with a UVSJ, insert the HLSJ as the high pass filter, then feed filtered signal into the amp.

[rabbit73]

Originally Posted by Flint Ridge

The fog is kind of clearing now with the concept of the HLSJ to truly scrub out those signals.

ADTech made some measurements of 2 HLSJs and I added some notes to the results to help me understand them. This might help clear the fog:



ADTech quoted:

An HLSJ will pass EVERYTHING below the cutoff frequency (down to DC) through the "LOW" port unless it has a capacitive input on the LOW port. Neither of these two devices use that layout.

You just have to know the relative transition frequencies to select which might be appropriate.

I tested the Holland and Pico/Tru-Spec HLSJ devices a while back. The Holland starts to roll off between 100 and 110 MHZ with about 3-4 dB IL at 108 MHz. The Pico/Tru-Spec started rolling off between 110 and 120 MHz.

http://www.digitalhome.ca/forum/81-o...ml#post1396123

comments by holl_ands:

ADTECH measured the Frequency Response for both the Pico-Macom and Holland versions of the HLSJ (Hi-VHF/Lo-VHF Diplexer).

On the Lo-VHF Port, Insertion Loss for the tested Holland HLSJ unit was very low on Ch2-6, increasing across the FM Band to about 3-4 dB Insertion Loss on 108 MHz (Top of FM Band), whereas the measured Pico-Macom unit only had about 1 dB Insertion Loss.

On the Hi-VHF Port (actually Ch7 thru UHF), the Holland HLSJ had at least 25 dB Insertion Loss across the FM Band, increasing to as much as 40 dB in the upper part of the FM Band, whereas the measured Pico-Macom unit had at least 28 dB Insertion Loss, increasing to as much as 40 dB in the middle of the FM Band and reducing to about 30 dB on 108 MHz.

So YES, they are both effective as FULL BAND FM FILTERS.....but they also Eliminate Ch2-6 if that is an issue for you.

http://www.avsforum.com/forum/25-hdt...l#post30726945

Holland specs:
http://www.hollandelectronics.com/ca...-Diplexers.pdf

There is a long thread here about FM filters:
http://www.avsforum.com/forum/25-hdt...fm-filter.html

post #23 of 124 has curves of the MCM FM Filter:
http://www.avsforum.com/forum/25-hdt...l#post29973297

http://www.mcmelectronics.com/produc...-FM-88-/33-341
.

[Flint Ridge]

Ok, let me make another run at this with a bit more understanding and more clarity.

As my UHF and VHF antennas will be separated by necessity by about 20' I'll have to amp them individually. The UVSJ that I have is a Blonder Tongue and feeds power on the VHF side for reference.

So...

UHF #1 - KT-500 #1 - power inserter #1 - UVSJ "A" (High Side)

VHF #2 - (HLSJ in on High side - out on combined, terminate Low side) - KT-500 #2 - power inserter #2 - UVSJ "A" (Low Side), then the combined signal out of UVSJ "A" leads to TII Lightning Surge Protector w/ Ground Block feeding main incoming coax for viewing enjoyment.

Better?

[rabbit73]

Power inserters #1 and #2 are outside?

If the UVSJ passes power on the low side, why is the power inserter for the VHF preamp between the preamp and the UVSJ?

[Flint Ridge]

... Well yes, the inserters are outside but in an enclosure at the base of tower with power and a ton of other "stuff".

I'll have to go see about re-cabling to place the VHF inserter on the house side of the UVSJ. Is there a worthwhile reason for me to get that switched around now?

[ADTech]

As my UHF and VHF antennas will be separated by necessity by about 20' I'll have to amp them individually.

You're probably making this much more difficult than it needs to be. I doubt you actually need to amp UHF and VHF separately. 20' of RG6 coax only attenuates VHF signals by about .6 dB which is pretty close to nothing in the grand scheme of things. Based on your chart, KOMU and KRCG should have ample margin to tolerate that little extra bit of insertion loss. Plus, you completely omitted the factor that you'd then need TWO HLSJ devices as high pass filters, one for each amp.

Here's my specific recommendation:

Mount UHF antenna where it needs to be and the VHF antenna where it needs to be. Run a cable from the VHF antenna to the UHF antenna's location and combine them using the UVSJ. Run that into the capped HLSJ to eliminate everything below channel 7, then immediately amplify with the KT500. Install the power inserter at the other end of your lead-in and you're done.

Your proposed scheme adds the cost of a second amp, a second cable run, a second FM filter, and double the amplifier power consumption all in a effort to save 6/10ths of a dB of signal on the VHF band. Therefore, I have to ask you, do you really need that?

Follow the K.I.S.S principle.

[Flint Ridge]

ADTech, good suggestions/recommendations, I always respect your opinion. Probably should have thought of that before now, but alas I did not. The 20' separation is up. As in to accomplish that my bellybutton will need to be above my 80' tower and after I make my way around 7 different receivers/dishes/mounting hardware etc.



So I land in the complicated category.

Now trying to wrap my mind around the 2 HLSJ's. So, your suggesting that the UHF needs this also? I will have a FM filter of unknown capabilities in the KT-500 on the UHF and that will also pass through the High side of the UVSJ, after it is amplified. (certainly not as good as your suggestion) Which I take as a bad thing. I have a similar setup on a 60' tower all UHF without VHF and UHF antenna with the KT-500 is 95% on WDAF/34. So, I have to hope my more powerful new UHF with higher gain peaked on that channel will work. It is all theory for now.

[ADTech]

What are you using for your UHF and VHF antennas respectively?

Strong FM signals will still be picked up by almost any antenna. The 91XG, for example, with its 8' boom, picks up a LOT of FM from the side via non-intuitive means.

Suppression of extraneous signals must be done PRIOR to amplification, otherwise, they can either mix with other signals (creates unwanted products) or drive the amp into compression which reduces the amp's sensitivity to weak signals.

[Flint Ridge]

That makes things more clear now. I had ordered an extra HLSJ anyway. So I will need to cut that in ahead of the UHF outside that weatherproof amp enclosure and protect with mastic tape, a pain yes, but doable. What I need is decent weather.

Antennas - well... the UHF is a DBGH20 optimized for max gain on 34 with a null for 14 at the rear with gain lobes for 17 & 22 at the rear, so I can only have one UHF and get signals in both directions. The UHF is a GH6. These are of the 4NEC optimized optimized type via nikiml's site.

UHF - as seen on top of picture
http://clients.teksavvy.com/~nickm/dbgh_u.html

VHF - is complete and awaiting mounting
http://clients.teksavvy.com/~nickm/gh_n_vV.html

[ADTech]

With those antennas, you'd have to work out what they might do with nearby FM, there isn't any data on that.

[Flint Ridge]

Agreed. I'll put it ahead of the amp on both.

I can see about running a freq scan on the lower ranges to see what they might be susceptible to.

[Flint Ridge]

On the UHF gain to the rear is higher by about 1 db. That would be in the general direction of the most powerful FM, at that freq of 92.3 I would be around 3.86 gain max, it will be off to the side of that, but I need to look at the max.

On the VHF, max gain is also off the backside and in this case it seems pretty blind to FM. Max gain projected on that signal of -10.5 The most it does is -8.51 around 80mhz at the rear.

My way of thinking is I need the filter more on the UHF antenna ahead of the amp, over the VHF.

Thanks again. Must say the 4nec modeling is kind of fun when you can check all the spec's out yourself.

[ADTech]

UHF reception patterns do not translate to FM patterns as the antenna will often have very different behaviors outside of its design band. They may well be very, very different .. or not. The antennas probably need to be modeled across the FM band to see what to expect... or just install the high-pass filters and don't worry about it.

[rabbit73]

Nice tower photo, Flint Ridge.

It isn't just the fundamental overload from KSDL at -14.0 dBm you need to be concerned about, but also second harmonics from FM signals in the VHF-High band:

Testing for DTV Interference
http://www.tvtechnology.com/expertis...ference/202503