Building a Reflector-less SBGH?

[shmish]

Hi,

I'm looking into making a SBGH for a metro application here in Vancouver (thanks to Stampeder for pointing me in this direction). The idea would be to start without a reflector. In this situation, do I need to worry about grounding the antenna? For example, suppose I fasten the antenna arms (dipole?) to some treated wood and put it on my roof. I think the only ground required would be to use a coaxial grounding block near the antenna. Secondly, if I'm not using a reflector should I expect a change in performance based on what I secure the arms to? For instance, I could attach the arms to a 1" pvc mast, a 1x1 piece of wood, or a piece of 1/2" plywood that is 30cm wide.

thanks!

[stampeder]

shmish, what I would try is a simple standalone mount, sort of like how John SE Texas mounted a reflectorless SBGH beneath his VHF/UHF combo yagi (look closely at photo) in Post #642:

http://www.digitalhome.ca/forum/show...&postcount=642

And you can see EGC's simple mount in Post #446 that you could adapt without a reflector:

http://www.digitalhome.ca/forum/show...&postcount=446

As for grounding, you'd run the coax from the antenna to where it enters your house, at which point it would connect through a ground block before entering, just as you said. Regarding the mast material, I believe 300ohm has counselled against the potential for moisture in wood, and with you living in Vancouver with all the rain we get I second that!

[shmish]

Thanks, that was pretty much what I was thinking. I could fasten some 1" pvc to the back of the wood (after weatherproofing the wood of course), and then fasten the pvc to the chimney.

[shmish]

Sorry if I'm a bit thick on this...

I believe 300ohm has counselled against the potential for moisture in wood, and with you living in Vancouver with all the rain we get I second that!

Is this regards to lightning, or contact of the antenna with wood? I think it's best to insulate between the antenna and the wood with some styrofoam, but would the wood mast (or wood/pvc mast) need to be grounded for lightning?

lol, I think I'm having a hard time keeping my questions on-topic in the correct thread.

[Onslow]

I found what 300ohm was saying by putting "wood moisture" into the Search This Thread tool. Moisture in wood interferes with signal.

[shmish]

Yes, that's what I got too along with the tip on styrofoam. I just want to make sure I understand the grounding issues.

thanks!

[300ohm]

Thanks, that was pretty much what I was thinking. I could fasten some 1" pvc to the back of the wood (after weatherproofing the wood of course), and then fasten the pvc to the chimney.

I think youre missing the point about the plastic / wood. Damp wood is conductive, and will somewhat change nature of the antenna if the elements touch the wood. The elements should be resting on the non-conductive plastic. Technically, styrofoam is best as far as conductivity is concerned, but is not durable at all.

If you must use wood for the mast, then the cross strips should be a strong plastic. That can work out OK if you use a 90mm or so feedgap SBGH on the end of a two by board.

Like stampeder said, the coax from all outdoor antennas need to be fed into a proper lightning arresting grounding block before going inside. Normally a metal mast would be grounded too, but with wood or plastic, there is nothing to ground.

[shmish]

Got it, thanks. I was also thinking about cutting some pieces of 1/4" hdpe from an old kitchen cutting board, and using this as standoffs from the wood.

cheers

[shmish]

Hi,

For the 44mm width between the elements, is this the actual distance between the array elements, or would this be the theoretical distance between the elements if they were bent without any radius? Please see my picture below, it shows an exaggeration of the situation where the elements are bent with radius. It's not a huge difference but when we are trying to keep tolerances below 2mm, things add up pretty quick. Using 1/8" dia aluminum rod stock, I believe I was able to hand bend with 2mm radius. In hindsight, a hammer and clamp/vise would result in quite a tight bend. Perhaps that will be for next time.


thanks

[300ohm]

For the 44mm width between the elements, is this the actual distance between the array elements, or would this be the theoretical distance between the elements if they were bent without any radius?

It is center of wire to center of wire at the closest point is how its modeled. A wider feedgap than the original 44mm feedgap will give a little more gain (up to a point where the elements would de-couple). 85mm to 95mm feedgap is a good range. A study of it was done way back about page 8 of this thread. The most important thing would be to make sure the gap is consistantly the same at all three points (the > < in the antenna).

Keep in perspective, the 2mm tolerance is so your antenna matches the model. If its out more than that, that doesnt mean the antenna wont work at all, but that it will work differently, for good or bad.

[shmish]

Here is some info on my temp build. It is reflectorless. The array is 1/8" Al round from Metal Supermarkets. The spine of the antenna is a piece of 3" western red cedar that I stained with polyurethane. Standoffs for the array were cut from an HDPE chopping board. The feedpoints (I don't know if that's the correct term) are connected with 1/4" brass screws. I've read about better compatibility with zinc or galvanized screws to match the aluminum array but I haven't (yet) found small plated screws. As well, the balun I purchased at Home Depot has really small spade terminals. The gap on the spade is 1/8" which means that really only a #4 brass screw will work. A #6 would will work if the spade fits in between the threads. I'm going to go to Rona tonight and see if they have a different balun or maybe I'll pick up smaller screws. I don't want to hack the spades unless I really need to, the balun is the most expensive part so far.

One issue that I didn't really think through is how narrow the wrc board is. I should have grabbed a wider plank from my scrap pile. As it stands, the feedgap is only 40mm. That's okay, I'll give it a try. I also cut some wider HDPE pieces. If I want, I can screw these to the wrc plank instead and have a much wider feedgap, up to 90mm. The other problem, of course, is that I need to sweep the floor and not take pictures of my socks. I'll update my progress probably later tonight once I get the balun sorted. First tests will be done indoors. Any suggestions on mounting to a chimney are appreciated. I'm planning on getting some 3/4" conduit that can mount to the back of the wrc, and then I can maybe use some clamps and concrete anchors to attach the conduit to the chimney. I found some nice looking clamps and straps on the web (via ebay) that I think would be great. The kit was only $10 but the darn shipping was $25 UPS, which means another $30 brokerage fee!

[CSaanich]

Originally Posted by shmish

Any suggestions on mounting to a chimney are appreciated.

I used a couple of ratchet straps like the kind used to tie down product on roof racks, etc. A small block of wood provided an offset for the mast near the top where the chimney lip is. The mast pipe goes down to the roof and is centered in a concrete post block.

Here's the link to the pic of the antenna(s) with straps:
http://i525.photobucket.com/albums/c...8/162_6205.jpg

[300ohm]

One issue that I didn't really think through is how narrow the wrc board is. I should have grabbed a wider plank from my scrap pile. As it stands, the feedgap is only 40mm. That's okay, I'll give it a try. I also cut some wider HDPE pieces. If I want, I can screw these to the wrc plank instead and have a much wider feedgap, up to 90mm.

No, dont get a wider board, just cut some wider HDPE pieces. You dont want any part of the antenna metal touching the wood, even with thru bolts.

I've read about better compatibility with zinc or galvanized screws to match the aluminum array but I haven't (yet) found small plated screws.

Keep looking, theyre more common and cheaper than brass bolts and screws.

Any suggestions on mounting to a chimney are appreciated.

Some Radio Shacks still carry chimney mount straps, so its much better to look for them locally. The nylon straps CSaanich mentioned may last a couple of years outdoors.

[CSaanich]

Originally Posted by 300ohm

The nylon straps CSaanich mentioned may last a couple of years outdoors.

Yeah, I should have added that this method is only being used as a temporary setup while testing and retesting my rig. A more permanent solution like the metal CM3080 brackets would be needed

[shmish]

Yeah, I don't know what I was thinking when I passed by the zinc screws... So I picked up some #6 screws, connected the antenna to my balun and tested it out inside (it was 10pm at this point). The SBGH seemed to have a stronger signal than a Silver Sensor clone I'm borrowing. I can't give too many details because I was having a lot of problems with the tv tuner.

No, dont get a wider board, just cut some wider HDPE pieces. You dont want any part of the antenna metal touching the wood, even with thru bolts.

yup, I already have the pieces cut. In hindsight I'm not sure why I started with the shorter HDPE pieces. It was a bad idea for reasons already mentioned and pointed out.

One thing I'm not sure of is the amount of contact needed at the feedpoints. The screw itself is not touching the array. The contact is from the aluminum rod to a large washer to the small washer, then to the screw head and down to the balun spade. I have no idea if this causes an issue in signal strength? Maybe this is a dumb question, but is this something that I could test with my multimeter, by measuring the ohms between the contact points (the al rod and the balun spade)?