KGO (7.1) reception issue Palo Alto, CA 94301

[rohit]

Hi
I put up a DB4 antenna 18ft off the ground to see what I can get -- I can reliably receive all channels listed @ tvfool for my address (zip 94301) except KGO (7.1). The signal strength (as measured in arbitrary units by my sharp TV) is approx 25 units compared to 50 to 60 units for KNTV, KQED, KPIX, KTVU channels.

My question: Is this because of the channel KGO is transmitting at ? (perhaps DB4 has poor gain at this frequency?) OR is KGO transmitting at a lower power level since i get other channels transmitted from the same location (Sutro tower in SF) just fine.

[GroundUrMast]

You can get a great deal of useful information by running a TV Fool Report... http://forum.tvfool.com/showthread.php?t=4 It would help a great deal if you would post a copy.


KGO is transmitting on VHF 7. The DB-4 is an excellent UHF antenna, but it does not cover the VHF band... That's by design. I think you can improve your VHF reception quite easily, but without a TVF report, I'll be guessing.

KNTV is on VHF 12 but is probably doing OK, in part due to higher signal level and the frequency is higher, closer to the UHF band the DB-4 is optimized to cover.

[John Candle]

As the other question askers do , Do This --> http://forum.tvfool.com/showthread.php?t=4

[mtownsend]

The DB4 is a UHF-only antenna (channels 14-69). It's not really designed to pick up VHF channels (2-13).

To pick up KGO (real channel 7), you can either add a high-VHF (ch 7-13) antenna to your current setup, or replace your DB4 with a "combo" antenna that can do both VHF and UHF. You don't have any channels in low-VHF (ch 2-6), so a high-VHF antenna is all that you would need. The Winegard YA-6713 and Antennacraft Y5713 are good antennas to consider.

If you want to add a VHF antenna to your existing DB4, you should combine them through a UVSJ diplexor (like this) as opposed to using a standard splitter/combiner. The UVSJ will merge the signal with minimal loss and mutual interference between the two antennas.

[rohit]

I did run a tvfool report before installation and thought since it is close enough (27 miles LOS) , the db4 might pick it up - - guess not.

the tvfool report is here: http://www.tvfool.com/?option=com_wr...3cf4e1caddd16c

also, http://www.choisser.com/channels.html shows that KGO 7 is transmitting with the lowest power among all the Sutro tower broadcast tenants - and they have an application for a 2 kW station on monument peak under consideration so things may improve.

there doesnt seem to be another worthwhile VHF station in the area so i think i wont go with a special antenna for KGO / (ABC) for now --

[John Candle]

The AntennasDirect C5 comes with a diplexer

[rohit]

I should clarify - the DB4 antenna I installed is ChannelMaster 4221 (bought at local Frys) -- does anyone know if it is cutting off VHF band @ the Balun?

I am receiving KNTV (NBC, Ch 12 210MHz) just fine - no pixellation etc.

Searching online for VHF Hi testing for CM4221/DB4, i found http://m4antenna.eastmasonvilleweath...Data/Data.html

The plot (scroll down to the last one) shows -15 dB for 4221 @ 180MHz. Dont know if its due to the design OR the balun cutting off lower frequencies. tvfool shows Noise margin @ 53 dB so it seems like I should still be ok with the following approx losses:
cable run (RG6 x 150 ft) attenuation = 5 dB
Connectors (3) excess loss = 3 dB
Antenna Gain (negative) = -15

Effective NM at tv = 53 - 15 - 5 - 3 = 30dB

unaccounted for: balun loss @ 180MHz and unless it is set up to filter out VHF and attenuates the signal another 20-30 dB ???

suggestions ?

[GroundUrMast]

A common matching transformer / balun is a broadband transformer with no intentional filtering capability.

As I said before,

KNTV is on VHF 12 but is probably doing OK, in part due to higher signal level and the frequency is higher, closer to the UHF band the DB-4 is optimized to cover.

You can substitute CM4221 for DB4 in my statement if you like.

Integral baluns on some antennas are strip-line circuits that are a bit more frequency selective. My Antennas Direct unit uses that design, still it provides a fair signal on 9 and 11 here in Seattle. (That's with 60 dB NM's though)

There is more to reliable reception than signal level. High VSWR (poor return loos) will cause signal to reflect backward in the coax, causing multipath interference symptoms. Using a UHF only antenna to receive a VHF signal guaranties high VSWR. The directional quality of the antenna is also going to be different as the frequency goes away from the design center.

[mtownsend]

Originally Posted by rohit

http://www.choisser.com/channels.html shows that KGO 7 is transmitting with the lowest power among all the Sutro tower broadcast tenants - and they have an application for a 2 kW station on monument peak under consideration so things may improve.

Watch out when comparing transmitter power. Low frequency signals propagate more easily than high frequency signals so it takes less power to cover the same amount of distance.

Low frequencies are also better at diffraction (bending over mountain tops), so they tend to do a better job of "filling in" areas that are behind terrain blockages.

As an example, take a look at the following two coverage maps.

Here is KABC's coverage map.

Here is KXLA's coverage map.

Both of these transmitters are on Mount Wilson overlooking the Los Angeles area. KABC is on channel 7 with 25 kW while KXLA is on channel 51 with 1000 kW. If you compare the two coverage maps, you will see that they both do an excellent job of covering everyone in the Los Angeles basin despite the 40-times difference in broadcast power.

KABC actually does a better job of reaching people in the high desert north of LA in places like Lancaster and Palmdale due to the better diffraction effects on channel 7.

Even though KGO's 23.8 kW *sounds* low, it is actually plenty of power for that channel. All of your Sutro channels are reaching your location with Noise Margins of about 50-60 dB, and KGO is in the middle of the pack at around 54 dB.

It all boils down to having the right antenna for the job. Your DB4 is probably getting around +10 dBd of gain on your UHF channels, but it's channel 7 performance is probably somewhere around -18 dBd (that's about a 28 dB difference compared to your UHF channels). You might want to take a look at this page, http://www.hdtvprimer.com/ANTENNAS/comparing.html, and scroll down to the section labeled "Using a UHF antenna for VHF". Adding a VHF antenna to your setup will probably get you back to around +7 dBd of gain on your VHF stations (a 25 dB improvement from where you are now).

[rohit]

thanks for the replies - poor SWR will certainly kill signal in a hurry on this antenna and power is likely not an issue (VHF propagation is better vs. UHF as pointed out above).

i really dont want to add an antenna just to pick up 1 channel (sorry ModernFamily on ABC) -- reading around on digitalhome.ca and other forums, there are a few CM4221HD hacks mentioned (http://www.digitalhome.ca/forum/showthread.php?t=104837) and I will try these to see if it brings in Ch7/KGO. The size of the reflector screen seems to matter (current 24" may not be sufficient to reflect VHF frequencies, quarter-wave @ 180MHz = 40cm). UHF signals appear strong enough that losing some gain at UHF with a larger reflector may still be ok.

From http://www.hdtvprimer.com/ANTENNAS/comparing.html it appears CM4228 may have much better VHF reception at the cost of increased size/cost.

[mtownsend]

Originally Posted by rohit

i really dont want to add an antenna just to pick up 1 channel (sorry ModernFamily on ABC)

Perfectly understandable.

Originally Posted by rohit

From http://www.hdtvprimer.com/ANTENNAS/comparing.html it appears CM4228 may have much better VHF reception at the cost of increased size/cost.

Yes, it does. Note the long, continuous rods used in their reflector. It's *ok* for situations with really strong VHF signals, but it's still not as good as even the most basic VHF Yagi antennas.

Although instead of swapping out a DB4 to put in a CM4228, I'd opt for simply adding a YA-6713 to the DB4. The two antenna setup is a better all-around solution for your situation if you're going to go through the trouble of spending some money and reconfiguring your antennas.

[GroundUrMast]

For what it's worth...

Theoretical gain: 0 dBd, YMMV

[rohit]

Tried the following things:
- made the 4221 modifications as suggested in link above (digitalhomes.ca) - made little difference to VHF, improved UHF channels approx 10-20% as measured by tv in arbit units.

- made a larger reflector (36") for 4221 - didnt make a difference.

- As I was trying the various combinations, I had moved the antenna down about 4 ft - -that brought in KGO/Ch7 nice and clear -- and UHF channels were down but still receiveable. Moved the antenna around to see if that made a difference -- turns out KGO/Ch7 is received better when antenna is not pointing right at the tower (323 true) - highest reception @ approx 318. UHF channels degrade but still receivable.

So I left it there for now and Ch-7 is ok so far with the signal varying by approx 50% over minutes (fading likely). It is a cloudy day in the bay area so that may have something to do with better signal reception on VHF (trop scatter ?).

If it is still not reliable, I will try the half wave dipole + combine the signals. I did notice that when I was near the antenna, the VHF reception improves by approx 20%.

[rohit]

Ch 7 kept fading in/out at the lower antenna position so tried the dipole alone as suggested by 'GroundUrMast'. Interesting result - still no Ch-7 though the dipole pulls in Ch12 (KNTV/NBC) very well. So it may just be a poor reception area for Ch7 (antenna location).

Combining Ch7 and UHF antenna (combiner says 5-1000 MHz bandwidth, 3.5dB loss so should be ok frequency-wise) didn't improve anything.

So I may have to pony up more bucks and either try out a better antenna (doubtful it would work given that the dipole didnt work for Ch7) OR put up the antenna on the smokestack above the house and see if that location works better - professional installer required, i dont have ladders that go that far up and I wouldn't feel safe doing it either. Attic not an option unless I do a new cable run.

Thank you all for all the helpful suggestions and explanations - I learnt something new.

[mtownsend]

You can optimize the peak gain of the antenna toward lower frequencies by making the elements longer. 16.7" would favor channel 7 instead of channel 12.

A perfectly tuned dipole has a theoretical gain of 0 dBd if everything was perfectly efficient. Baluns are never 100% efficient, and there are bound to be slight impedance mismatches, so your net effective gain is probably more like -2 dBd.

The multi-element Yagi antennas like the YA-6713 and Y5713 are able to achieve about +6 to +7 dBd, so I do expect them to perform better than a simple dipole. They also have better directionality, so they might cut down on some multipath interference if that is one of the factors causing you to lose channel 7.

To combine the VHF and UHF antennas, you should use a Pico Macom UVSJ or equivalent. This should limit your combiner losses to only about 0.5 dB, and it will prevent mutual interference between the two antennas.

[ADTech]

The above dipole is correctly scaled if you are using a 1:1 balun. Since most available baluns are 4:1, it is necessary to increase the length of each dipole by about 2 1/4" for overcome the impedance mismatch.

Our design engineer recently whipped up a simple dipole that we are considering making available as an add-on for our ClearStream UHF antennas. His elements are each 16.25" from the balun terminal to the tip with a gap of about 1.5" between the balun terminals.

In Palo Alto, expect strong potential interference from FM stations off of San Bruno. Use either an HLSJ with the "L" port terminated or an FM filter that attenuates the entire FM band (ours does).

[rohit]

i am using a $2 balun bought from Frys (300 Ohm to 75Ohms) - thats the only one they carry.

even with longer elements, i cant get Ch7, Ch12 reception is really good even with longer dipole length.

i am beginning to think I may be in a strong fade/rf-shade area for VHF. There is a tall (10 story) building about 400 mts away at 345' bearing (true) that could be a factor as well. No other tall buildings for 4 blocks.

[GroundUrMast]

I agree with the advice offered thus far. (Consider adding a high band VHF antenna http://www.solidsignal.com/pview.asp...u=716079000987 Consider the addition of an FM trap. Use a UVSJ to combine UHF and VHF antennas, not a reversed 2-way splitter.)

[rohit]

thank you all for all the information and suggestions - all very helpful !

i think the right combo for this situation/issue is a separate Yagi Uda VHF antenna and to do it right, add FM-rejection + UHF/VHF combiner.

rohit

[paloaltodad]

Thank you all very much for this thread!

My DB2 antenna antenna received ten UHF stations with better than 90% (as reported by my SiliconDust tuner) - but (RF) 7 and 12 were unwatchable.

Following the advice in this thread, I added a Y5713 VHF antenna and UVSJ (and also a 6ga copper ground :-) and now signal strength on 7 is 95% up from 40%, and on 12 90% up from 75%. Thanks for the terrific and very generous help!