First antenna build problems....

[FNH]

234/frequency.

234/26.550=8.81 or roughly 105" or 2.667 meters

That is the length of your vertical and also your ground radials.

three or four radials are all you need to make this ground plane efficient.

Hi.The only thing I can see is that the ground radials are half the length they should be.Try extending at least 3 of them and see what happens.If you don't have any more tubing use wire with a insulator on the end and tie it down to the roof, (like guy wires) Can you test swr a bit higher and lower in frequency to see if there is a dip outside of the band your using.

Just had a thought, check continuity between tube sections. anodizing isn't the best conductor

There are different types of anodizing. Some are insulators and some conduct. No idea what to call either so I guess try it and see.
- 'Doc

Hi Guys,

Thanks for all the additional info.

I am using an Yeticom Optima which will cover 24.500 - 29.999. Re the anodising: I'm not sure what grade of anodising I have got here, but all the clamp screws are stainless with either the serated cup ends or ground to a point and really bite into the alloy through the anodising.

I have raised the riser tube up as far as it will go on the angle bracket. Then extended the radial tubes by threading the ends and using a threaded stud to attach another tube. They are a bit short at 2.635m, but better than they were origionally. I checked the SWR after doing this and it had dropped to 2.8-3.0. Some how I must have misinturpreted the lengths and thought the radial length was acroos the whole assembly and not just one side. Then I bent the radial tubes down, they're not 45 degrees because they'd hit the roof, but maybe 30 degrees. I checked again and the SWR is in the 1.1 - 1.5 area. So a vast improvement on yesterday. I just need to have a play and get the high and low ends of the range balanced better and we will be in business. I can get the SWR lower, but they are still not balanced. I just need to play around a bit more.

Is it okay to leave the SWR meter in line or is it best to remove it once I have finished adjusting?

Thanks again.
FNH

 

[vkrules]

Glad you got it going.Leave the meter in line for peace of mind.Try 27.385 lsb.States and nz have been coming and going all day. 73s Mark

 

[wavrider]

Glad you got it working for you.

Make some contacts and have fun.

 

[FNH]

Glad you got it going.Leave the meter in line for peace of mind.Try 27.385 lsb.States and nz have been coming and going all day. 73s Mark

Glad you got it working for you.

Make some contacts and have fun.

Cheers guys. Been scanning around all evening, but things have been pretty quiet. Will keep at it over the coming week and see what happens.

Glad I can put the ladder away now though. I reckon I lost a couple of kilo's this last week, with all the 'step aerobics' I've been doing

FNH

 

[Marconi]

FNH, try raising the antenna up so the ends of the radials are at least 10' feet above the metal roof, and try and get the radial angle more steep, maybe down to 20* - 25* degrees from the mast if possible. Then you should be really good to go if you don't get bad reflection off the roof.

Corrugated metal looks ideal for mounting an antenna above, but that may just be the way it looks to the user, and not to the antenna. My experience says this roof material can be another problem that might not appear to effect the match, but might still seriously ill-effected the antenna response in a destructive way. I found such installs seriously attenuated real performance, and I think even 20' higher might get you well into the open enough to get better contacts. If we could see RF this would be easy to figure out, but let your senses guide you on the roof deal.

Also regarding the length of the stinger. I can't tell how long the feeding line setup that is exposed out side of the coax really is, but with that type of feed point construction you have to consider that the exposed wire outside of the coax is radiating, big time at that point, and IMO long is not good. It will be shielded inside of you mast pipe, but again it is hard to really tell what is going on in this case. If this length is no more than about 3" inches, then I wouldn't worry about it, even though you'll still have to figure it's a big part of the matching length for the radiator.

BTW, this 1/4 wave with proper horizontal radials should naturally show you a resistive match of about 25 ohms near the base, and angling the radials down tends to raise this impedance reading up not down.

The radial length should also be about as long as the radiator, given the diameters are about the same and the closer this is the better the balance is at the feed point...which is important also.

Good work and design, make the elements a bit larger the next time, and even old Mother Nature will talk bad about you.

Welcom to the forum, and keep us posted on your contact work, OK?

 

[FNH]

FNH, try raising the antenna up so the ends of the radials are at least 10' feet above the metal roof, and try and get the radial angle more steep, maybe down to 20* - 25* degrees from the mast if possible. Then you should be really good to go if you don't get bad reflection off the roof.

Corrugated metal looks ideal for mounting an antenna above, but that may just be the way it looks to the user, and not to the antenna. My experience says this roof material can be another problem that might not appear to effect the match, but might still seriously ill-effected the antenna response in a destructive way. I found such installs seriously attenuated real performance, and I think even 20' higher might get you well into the open enough to get better contacts. If we could see RF this would be easy to figure out, but let your senses guide you on the roof deal.

Also regarding the length of the stinger. I can't tell how long the feeding line setup that is exposed out side of the coax really is, but with that type of feed point construction you have to consider that the exposed wire outside of the coax is radiating, big time at that point, and IMO long is not good. It will be shielded inside of you mast pipe, but again it is hard to really tell what is going on in this case. If this length is no more than about 3" inches, then I wouldn't worry about it, even though you'll still have to figure it's a big part of the matching length for the radiator.

BTW, this 1/4 wave with proper horizontal radials should naturally show you a resistive match of about 25 ohms near the base, and angling the radials down tends to raise this impedance reading up not down.

The radial length should also be about as long as the radiator, given the diameters are about the same and the closer this is the better the balance is at the feed point...which is important also.

Good work and design, make the elements a bit larger the next time, and even old Mother Nature will talk bad about you.

Welcom to the forum, and keep us posted on your contact work, OK?

Thanks Marconi, I was thinking late last night about seeing if I could raise it up some, to at least get clear of that chimney. I'd then have to build some sort of platform to straddle the ridge to put a ladder on, so I cn tune it though. It's too tall for me as it is and I'm standing on tip toes, on a pallet, on blocks of wood at the moment! I'm pretty sure the local fishermen are running sweapstake to see what time I fall off! But, yes I think more height is the anwser.
Stinger. If a stinger is the sheathed inner core, where it leaves the outer braiding and then attaches to the vertical, then it is 2.5" - 3" long.
Cheers
FNH

 

[nosepc]

The radials are erroneous to work as a ground plane.

They need 5% more length to function as such.

)

 

[The DB]

The radials are erroneous to work as a ground plane.

They need 5% more length to function as such.

)

5% to short??? Really?

Any which way you look at it the ground plane will be inadequate, you won't notice any difference from adding that extra 5% to their overall length...


The DB

 

[Marconi]

I was suggesting that the 2.5" - 3" will have some effect as part of the overall radiator length, and the top element (stinger) being the adjustable part.

 

[FNH]

I really appreciate all the comments guys.

As there isn't a great deal going on around here on 26.mhz I have tuned it in to get the best SWR on 27mhz. I can alway stick up another old mobile antenna and tune it to 26mhz at a later date and use a switch to talk with the local guys. I have made an extension pole to raise the antenna up passed the chimney and it is ready to be collected from the zinc platers when I'm next in town. I've also made up some new one piece radials of the correct length and they'll go to the anodisers with the next work order. So progress is happening.

Next question...

I am worried about strong winds and although I have braced the base and have guy ropes on the riser tube, I am still worried about the antenna itself. I wondered about turning a plastic 'dohnut' with 4 holes at 90 degrees, that slipped over the upper vertical piece and sat above the lower vertical. I can then attach 4 nylon guy ropes to the dohnut, so that the antenna is supported midway up. Even if I used non conducting materials, would there be an adverse effect on the efficiency?

TIA FNH

Dohnut would sit above change in diameter in the picture below

 

[Marconi]

FNH, I don't think the donut idea will hurt you, but you could just as well put some loops of Poly type lines over the top section and let it rest against the larger part of the radiator. You would accomplish about same thing to guying the radiator with the donut.

Go back and read my post #20, where I talk about two guys I know that mounted their A99's on top of corrugated metal roofing. One was about 4' feet high above his 18' foot roof. Their antennas were dead in the shortest and longest directions that the metal ran.

One guy moved his antenna to the far end of the building near the edge, and saw improvement away from the building only. He ultimately got smart and moved his antenna to a tower on the ground out back, and then he was fine.

I'm in SE Texas, and I've been hearing Australia, New Zealand, and there abouts pretty regular for the last month or so, but they are not in there long, mostly in the mid morning, and late afternoon. From the way those guys I hear talking, there must be some pretty good traffic down there, and BTW I work a Starduster at 46' feet to the hub, and a Sirio Gain Master at 39' to the base, and I see first one then the other making the best contacts.

At my location my SD'r is the quietest by a good measure when conditions are moderately to really quite.

Good luck

 

[W5LZ]

Will that donut and guys change things? Not enough to make any huge differences. A little bit, maybe, but nothing you can't compensate for.
- 'Doc

(Talk to it sternly! That helps sometimes...)

 

[FNH]

I've added the extension pole today to raise it up a bit more and added the extra guy ropes with the nylon ring as discussed above. Unfortunately the only rope I found that was the right size and that stated it was UV resistant was bright yellow :blink: , but the whole thing is pretty sturdy now and should cope with a considerable wind. My SWR has raised a little after adding the extension, but the wind got up before I got a chance to re-tune it, so I'll have to wait for another calmer day, before I can judge the results.

My property is pretty small and my house and business premises take up two sides and one corner of the property and the other two sides that are more open and would be suitable for a tower have roads and power lines running along them. So this is what prompted me to try and build this off the roof.

FNH

 

[vkrules]

Skip has been rolling in for the last 3 days, NZ always does well. best time for you to listen would be about midday, after dark you will not hear much. Try 27.385 lsb,I think only NZ uses 26 mhz. PS I think your house will blow over before that ant will.

 

[Mudfoot]

Pythagoras would be proud!