300 ohm wire, max length??

[jrgagne99]

Did you locate the pre-amp at the antenna mast?

[rabbit73]

Originally Posted by BigHenry

The new pre-amp is installed.....The 7's are still out with "No Signal".

WDBJ virtual channel 7.1 has an "R" in the right column of the rabbitsears report which indicates reduced coverage. They might still be using their low power STA transmitter with a directional antenna. It's only 10% of the normal transmitter power.
https://www.rabbitears.info/tvq.php?...ms&facid=71329

[BigHenry]

Yep, the black plastic pre-amp is mounted on the mast very close to the antenna frame. I used a short (18") piece of RG-6 coax connecting the balun to the pre-amp.

Yes, channel 7 is only broadcasting at reduced power. They have been trying to get back to full power since September. I call them about every 2 weeks to get a status. They hope to resume full power by end of December.

[rickbb]

Originally Posted by BigHenry

Yes, channel 7 is only broadcasting at reduced power. They have been trying to get back to full power since September. I call them about every 2 weeks to get a status. They hope to resume full power by end of December.

That's good to hear, I miss having them as a backup to my local, (and sometimes crappy), CBS station.

[BigHenry]

rickbb,

I just saw WDBJ's website and they announced that the new transmitter is operational as of today.

I just called Shentel (our local cable company) inquiring about the price of basic cable. I damn near bit my tongue in two when the sales droid said that with taxes, surcharges, graft, et al, the cost would be a bit north of $50 a month. I'm re-motivated to keep trying OTA. What else should I try? Here are a few ideas:

1. Find a way to shorten my CM 94444 leads so that the balun still hangs down.

2. Try a new dropline from the antenna into the house.

3. Could my antenna have something physically wrong, bent metal or a short?

Any other suggestions?

[jrgagne99]

How old is the dropline down into the house? Have you tried taking a portable TV right up there to the antenna to check reception there? No pre-amp is needed for this test.

I tend to doubt that a bent element would be affecting your reception so adversely.

[BigHenry]

Hey, some good news!

As suggested by rabbit73 in an earlier post, I figured out a way to shorten my balun leads to 2.5" while still able to mount the balun. As if by magic I'm now receiving solid signal strengths on ALL original channels (7.1, 7.2, 10.1-10.5, 15.1-15.4, 24.1, 27.1-27.4, 38.1-38.6.

I would like to thank all who helped me out, with a special thanks to rabbit73 for the balun testing leading to the discovery of an optimal balun lead length.

My balun attachment is rather ugly - the balun sits right alongside the main horizontal boom with one lead crossing over the boom, which looks OK, but the lead which attaches to the post on the same side of the balun is a bit compressed. Not as pretty as the long lead set-up where a lead comes down on each side of the boom, allowing the balun to sit on the centerline below the boom. But it's the results that count!

Frankly, antennas, baluns/leads and RF frequency waves seem like voodoo to me. Could any of you experts on the forum help me understand why shortening balun leads so dramatically improved my reception. Does the orientation of the leads also have an effect? e.g. like the leads in close parallel proximity or spread apart as far from each other as possible.

Lastly, as long as balun lead placement won't mess up my reception, I am thinking of inverting the threaded studs to which the balun leads attach, so they will now be beneath the boom and once again the balun will hang down on the centerline below the boom.

Thanks,

Don

[Tim]

Originally Posted by BigHenry

Lastly, as long as balun lead placement won't mess up my reception, I am thinking of inverting the threaded studs to which the balun leads attach, so they will now be beneath the boom and once again the balun will hang down on the centerline below the boom.

My rule of thumb has always been, "If it ain't broke, don't fix it."

[rabbit73]

Originally Posted by BigHenry

As suggested by rabbit73 in an earlier post, I figured out a way to shorten my balun leads to 2.5" while still able to mount the balun. As if by magic I'm now receiving solid signal strengths on ALL original channels (7.1, 7.2, 10.1-10.5, 15.1-15.4, 24.1, 27.1-27.4, 38.1-38.6.

I would like to thank all who helped me out, with a special thanks to rabbit73 for the balun testing leading to the discovery of an optimal balun lead length.

Thank you for the report of improved reception. I'm glad that my balun tests gave you an idea that helped. I hope the improvement will continue.

Frankly, antennas, baluns/leads and RF frequency waves seem like voodoo to me.

Antennas have always seemed like magic to me. I have been experimenting with them since I was 8; I'm now 86.

Could any of you experts on the forum help me understand why shortening balun leads so dramatically improved my reception. Does the orientation of the leads also have an effect? e.g. like the leads in close parallel proximity or spread apart as far from each other as possible.

A little bit of luck helped both of us.

The wire leads from the antenna terminals to the balun form a transmission line. The transmission line transfers the signal energy from the antenna terminals to the balun. If the design of the transmission line is correct, most of the energy will reach the balun.

The transmission line must have the correct impedance and length to match the output impedance of the antenna terminals to the input impedance of the balun windings. The impedance of the transmission line is determined by the diameter of the wires and the spacing between the wires. My thought was to make it look something like the spacing of 300 ohm twinlead. Some baluns actually have 300 ohm twin lead instead of two separate wires.

Lastly, as long as balun lead placement won't mess up my reception, I am thinking of inverting the threaded studs to which the balun leads attach, so they will now be beneath the boom and once again the balun will hang down on the centerline below the boom.

My reaction is similar to Tim's. Don't mess with it if it's working. If you do mess with it, make certain you can put it back the way it was. The studs will become part of the transmission line.

When I was young, I built an audio amplifier on a breadboard using two 6L6 tubes in push-pull in the output stage. It worked very well. I bought an aluminum chassis and mounted the parts on the chassis. It didn't work as well.

When I was older, I built a regulated power supply for an electronic flash using transformers connected to form a saturable reactor to regulate the voltage with a small DC control current. I built it in an enclosure that I had on hand; it worked very well. I built a second power supply in a fancy cabinet with a carrying handle. It didn't work as well, and I never could figure out why.

Thanks again, Don for your report of success with the balun. I consider it a Christmas present from you to me.

Best regards,
rabbit73

[BigHenry]

rabbit73, I too consider good reception to be a gift, so thank-you. But reaching 86 years and passing on your knowledge is your greatest gift.

I had no idea that impedance of the transmission lines played such a critical role in matching the output impedance of the antenna terminals to the input impedance of the balun. So if my transmission line was a little bit shorter, longer, fatter, skinnier or orientated differently I might have missed the optimal impedance and continued to have crappy reception. Now I understand the "luck" factor.

I'm going to go back to Channel Master and ping them harder to get the OEM built-in balun, because unlike my "lucky" CM 94444 balun the OEM was probably engineered to have the proper impedance. The trouble with the OEM balun is that the terminal studs don't seal well in the housing and are a moisture trap. But I can seal the studs with some silicone to keep out the water.

The saga continues...

[rabbit73]

Any plastic housing for a PCB balun will trap moisture from condensation. The best designed housings have "weep holes" at the bottom to allow the enclosure to breathe and drain moisture.

Older Channel Master preamps like the original 7777 with two antenna inputs not only had the connectors on the bottom for weather protection, but the bottom plate wasn't tight against the housing. This provided a small gap at the edges of the plate to allowed the enclosure to breathe.

The recent preamps sold by Channel Master have a gasket at the edge of the plate and coax connectors that have an internal seal which makes it difficult to insert the center conductor of the coax. I'm not convinced that trying to make the enclosure hermetically sealed is an improvement.

[BigHenry]

Well looky-here. In my shop trash, I found the old OEM balun that came with my CM-5020 antenna. I cleaned the water-induced oxidation off and will reinstall with better moisture resistance and some weep holes.

Actually it may be awhile before I install the OEM because the current CM-94444 balun is doing such an exemplary job. BTW, I cut open a spare CM-94444 and it is of the ferrite coil type, not a PCB. Based on what rabbit73 told us about getting balun lead impedance right, my gut feeling is that the OEM PCB balun is better tuned to the antenna than my lucky CM-94444. But I still don't want to risk it.

[rabbit73]

Originally Posted by BigHenry

Well looky-here. In my shop trash, I found the old OEM balun that came with my CM-5020 antenna.

Good find! Thanks for the photos.