Antennas Direct DB8 Hardware Hacks

[300ohm]

Does this mod add high vhf capability to the DB8? Would it help with channel 9, 10 and 11?

Basically yes, but it may have some dips in the vhf-hi gain curve on some of the vhf-hi channels. 36" should be good for channel 7, 28" should be good for channel 13, so extrapolate for other channels keeping in mind whats good for one may not be so hot for another.

[genseng]

Same question but I'm trying to improve channel 10 reception. Found something that indicates I would want 28.5 inches? Looks like 300ohm's photo has been removed so not 100% how I'm attaching these rods to the antenna.

[300ohm]

If its the picture Im thinking of, just put the longer 4 reflector rods directly behind each of the 4 pairs of bowties.

[chinadog]

I will be using a cm7777 with a vhf modified db8 ( as described in the previous post). What should I set the pre-amp to?

uhf and vhf
or just uhf?

[ADTech]

If you intend to attempt to use the DB8 for both, then set it to "Combined".

The "Separate" setting is usually for use when using separate U/V antennas.


I'll admit I'm somewhat skeptical that adding the rods to the DB8 reflector will improve VHF very much in light of the UHF balun that is used. I suspect you'd probably do better simply adding a VHF dipole with UVSJ (or the 7777 set to "Combine) on a stalk above one of the panels. It would look odd, but I suspect it would likely work better than the rods.

I haven't field tested the setup with rods, so I'm just going on intuition at this point. I'd certainly like to hear of any measured field results.

Cheers!

[threeflags]

Originally Posted by ADTech

If you intend to attempt to use the DB8 for both, then set it to "Combined".

The "Separate" setting is usually for use when using separate U/V antennas.


I'll admit I'm somewhat skeptical that adding the rods to the DB8 reflector will improve VHF very much in light of the UHF balun that is used. I suspect you'd probably do better simply adding a VHF dipole with UVSJ (or the 7777 set to "Combine) on a stalk above one of the panels. It would look odd, but I suspect it would likely work better than the rods.

I haven't field tested the setup with rods, so I'm just going on intuition at this point. I'd certainly like to hear of any measured field results.

Cheers!

Here's my experience. I did try it out with attaching 4 (32") rods on one set of bays (DB8). I was looking for better signal strength for channel 9. Overall the signal did increase slightly. It made just enough of a difference between able to watch with few and no drop outs vs. a black signal (no signal). Both bays aimed at the US transmitters. If I wanted to sacrifice some of my US signals ... the antenna would have worked great without the mod.

[300ohm]

I was looking for better signal strength for channel 9. Overall the signal did increase slightly.

Not bad, the 32" width was a quesstimate. I havent modeled the DB8 with the wider/longer rods. Even if I did, real world influences may throw the measurement a bit off because of the loopy nature of the gain curve when 4 bay and 8 bay bowties are used for vhf-hi.
Look at the gain curve for the CM4228 when its used for vhf here, and youll see what I mean :
http://www.hdtvprimer.com/ANTENNAS/comparing.html

What I would do is to use the telescoping rods from some thrift store rabbit ears and attach them to the reflectors behind the bowtie pairs. Using 28" for channel 13 and 36" for channel 7 as a rough guide, adjust the telescoping rods in 1/2" increments and test.

For me, scraping 2 thrift store rabbit ears is much cheaper than buying 4 - 36" lengths of 1/4" aluminum rod at ACE, Lowes or Home Depot, heh.

I'll admit I'm somewhat skeptical that adding the rods to the DB8 reflector will improve VHF very much in light of the UHF balun that is used.

The older DB8's didnt come with a custom balun, did they ?

[ProjectSHO89]

Older DB-series antennas used a standard balun, pretty much the same ones they've used on their V-series antennas and the C2V VHF dipole kit. The original DB8 used a standard splitter and two equal lengths of coax to combine the two DB4 half-sections.

They went to the PCB baluns sometime in late 2008, IIRC, for their UHF antennas.

[chinadog]

Just finished an install where I tested a DB8 combined with dollar store rabbit ear dipole via the UVSJ UHF/VHF Combiner . This was my homemade CV2 vhf adapter.
Instead of attaching the dipole to the back of the db8, I fastened it to the mast below the db8 instead
My objectives were to

1) snag fringe UHF signals in the south from Buffalo (90-115 miles away)

I got stable reception from rf44 (PBS17-1,17-2) and rf32 ( CW23-1) while other nbc,cbs, abs and fox come and go. There seemed to be only about a 1 db loss on the Buffalo stations when I combined the two antennas.

2) catch rf10 25 miles to the north (Barrie CTV2) and rf 9 30 miles to the south ( Toronto CTV). These are now rock solid. They were unwatchable using the db8 alone.

Unfortunately, my temporary test set-up came crashing down on the sidewalk! The two support braces joining the two halves of the d8 antenna bent, cracked and broke. Fortunately I had a spare xg-91 which I used instead for the install.

Dear ADTECH, would I be able to get a few spare parts from Antennas direct? If not, I now have two db4s to employ.

[roger1818]

Originally Posted by chinadog

Instead of attaching the dipole to the back of the db8, I fastened it to the mast below the db8 instead

That is exactly how I have been recommending people use the C2V upgrade kit to allow the dipole to be aimed separately from the UHF antenna. I have a picture in my mind of how this would be done, but have never seen it in practice. Could you could post a picture?

[chinadog]

roger1818:

"Could you could post a picture?"

will do.

[ADTech]

Dear ADTECH, would I be able to get a few spare parts from Antennas direct? If not, I now have two db4s to employ.

Call customer service at 877-825-5572 during operating hours.