OCF swr issue

[jazzboypro]

After checking for water and connection at the transformer and redoing the connection and made sure it's tight i juste lest the transformer on the chimney with the antenna in the air but really sagging and made measurements. SWR was acceptable on all bands. After raising the transformer to it's final position i'm back to square one with high SWR on every band. That got me thinking, the transformer has a hook on top as if it was made to be suspended. In my case what i did is i made an aluminum bracket and used 2 screws to secure the transformer to the bracket and then i attached the bracket to the mast with a U bolt. Can the bracket have an adverse effect on the antenna ?

 

[vagrant]

What material is the mast made of?
How close is the transformer to the mast and other metal parts?
Can you provide a photograph of the transformer in the position where the SWR is high? ( close up )

I use an 80-6m OCF with a Rohn 40’ mast, but the transformer is at least one meter away from the mast using a non-conductive conduit. I have always used it that way. I have never tested using it near the mast. The legs are definitely not near anything metallic.

Anyways, plenty of NVIS on 80m due to my low height. What distance were you planning to get on 80?

 

[jazzboypro]

What material is the mast made of?
How close is the transformer to the mast and other metal parts?
Can you provide a photograph of the transformer in the position where the SWR is high? ( close up )

I use an 80-6m OCF with a Rohn 40’ mast, but the transformer is at least one meter away from the mast using a non-conductive conduit. I have always used it that way. I have never tested using it near the mast. The legs are definitely not near anything metallic.

Anyways, plenty of NVIS on 80m due to my low height. What distance were you planning to get on 80?

I will get a picture as soon as the rain stops (even the weather is not on my side) The mast is actually a telescopic painter's pole. The first part is fiberglass and the telescoping portion is probably aluminum but i'm not sure. The transformer is attached to the metal part of the mast via the aluminum bracket that i made so it's close to the mast. The transformer is about a feet or 2 from a metal chimney but higher than the chimney (i would have to measure the exact distance. I mostly wanted 80 meter to participate il a few local nets that we have in the province of Quebec.

 

[jazzboypro]

@vagrant

The rain finally stopped and i went on the roof to take pictures of transformer.







Before going up, i verified SWR again and it was still high. When i went up to take the pictures i managed to place the coax further away from the mast and it appears to make a big difference in SWR. I thought coax was pretty much immune to this kind of thing.

 

[G7RUX]

@vagrant

The rain finally stopped and i went on the roof to take pictures of transformer.

View attachment 147124

View attachment 147125

View attachment 147126

Before going up, i verified SWR again and it was still high. When i went up to take the pictures i managed to place the coax further away from the mast and it appears to make a big difference in SWR. I thought coax was pretty much immune to this kind of thing.

Coax is very much not immune to interactions if it is not properly unbalanced; your antenna is forcing the coax screen to be part of the radiating structure so I would suggest checking the balun is doing what you expect it to and to consider a significant common mode choke close to the antenna feedpoint.

 

[jazzboypro]

Coax is very much not immune to interactions if it is not properly unbalanced; your antenna is forcing the coax screen to be part of the radiating structure so I would suggest checking the balun is doing what you expect it to and to consider a significant common mode choke close to the antenna feedpoint.

There is a choke installed but not near the feed point. I will try a few other things before moving the choke closer to the feedpoint as it would require to replace 2 sections of coax. I will do it as a last resort. In the mean time i went on the roof again to attached the coax to the mast. The antenna is now working on 10-12-20-30-40. Can't tune on 15 and 80 is usable until i hit 40W.

On 80 the SWR is worse when the internal tuner is on is this normal ?

I think my next move will be to find another kind of bracket to installe the transformer to the mast.

 

[prcguy]

When you tested the antenna on the chimney and it had a good match was the aluminum angle bracket on the transformer? I don’t think it’s a good idea to have a piece of metal bridging across the transformer and dipole legs.

 

[popnokick]

+1 on prcguy's concern about the aluminum bracket holding the transformer. Any way to eliminate that, such as using a non-conducting standoff arm on the painter's pole and hanging the transformer from the eye on the top of the transformer housing? This would eliminate the metal bracket, as well as get the transformer away from the painter's pole (which looks to be metal?).

 

[G7RUX]

@vagrant

The rain finally stopped and i went on the roof to take pictures of transformer.

View attachment 147124

View attachment 147125

View attachment 147126

Before going up, i verified SWR again and it was still high. When i went up to take the pictures i managed to place the coax further away from the mast and it appears to make a big difference in SWR. I thought coax was pretty much immune to this kind of thing.

Oh hang on a moment…I hadn’t looked at your photos due to low throughput here but how is that transformer attached to the bracket? It looks like the aluminium l-section is mighty close to the dipole attachment points…water sitting around there will likely cause what you’re seeing. If you hang the transformer from the bottom of that bracket you may well see your issues improve greatly.

 

[G7RUX]

There is a choke installed but not near the feed point. I will try a few other things before moving the choke closer to the feedpoint as it would require to replace 2 sections of coax. I will do it as a last resort. In the mean time i went on the roof again to attached the coax to the mast. The antenna is now working on 10-12-20-30-40. Can't tune on 15 and 80 is usable until i hit 40W.

On 80 the SWR is worse when the internal tuner is on is this normal ?

I think my next move will be to find another kind of bracket to installe the transformer to the mast.

Looking again at your photos I am pretty sure that bracket is where your issue is coming from. If you can replace the top bit of the pole with something non-conductive (a bit of fibreglass tube would be ideal) and hang the transformer from the hook on the top of it then things should be much better as you’ll get the metal and water bridge away from the feedpoint; higher power shouldn’t then cause it to flash over.

That said, RF on the coax screen will often cause SWR bridges to give quite misleading measurements so that might not help when actually transmitting.

 

[jazzboypro]

When you tested the antenna on the chimney and it had a good match was the aluminum angle bracket on the transformer? I don’t think it’s a good idea to have a piece of metal bridging across the transformer and dipole legs.

As a matter of fact the aluminum angle bracket was not attached to the transformer at this point in time.

+1 on prcguy's concern about the aluminum bracket holding the transformer. Any way to eliminate that, such as using a non-conducting standoff arm on the painter's pole and hanging the transformer from the eye on the top of the transformer housing? This would eliminate the metal bracket, as well as get the transformer away from the painter's pole (which looks to be metal?).

Yeah getting rid of the aluminum bracket was kind of my next step

Oh hang on a moment…I hadn’t looked at your photos due to low throughput here but how is that transformer attached to the bracket? It looks like the aluminium l-section is mighty close to the dipole attachment points…water sitting around there will likely cause what you’re seeing. If you hang the transformer from the bottom of that bracket you may well see your issues improve greatly.

The transformer is attached to the bracket via 2 screws (one on each side of the transformer.

Looking again at your photos I am pretty sure that bracket is where your issue is coming from. If you can replace the top bit of the pole with something non-conductive (a bit of fibreglass tube would be ideal) and hang the transformer from the hook on the top of it then things should be much better as you’ll get the metal and water bridge away from the feedpoint; higher power shouldn’t then cause it to flash over.

That said, RF on the coax screen will often cause SWR bridges to give quite misleading measurements so that might not help when actually transmitting.

Yes i am currently looking at other way to install the transformer

 

[jazzboypro]

I bought another telescopic painter's pole (the shorter one) i will remove the metal portion and keep the fiberglass part as a standoff and i will hang the transformer from below the standoff.

 

[jazzboypro]

Here is the "envisioned" setup. At the moment the antenna is not yet at it's final position i'm gonna run some tests first.

 

[jazzboypro]

Well..The antenna is at it's final position with the transformer suspended on a piece of fiberglass pole as in the picture above. Unfortunately i'm back to square one with high swr on every band. I've been looking at all my data and noticed that i obtained my best SWR when the antenna was not at it's final position and just laying on the chimney. That also coincide with the metal portion of the mast fully retracted in the fiberglass portion of the mast. I may need a mast that does not contain metal. Is there such a thing as a telescoping fiberglass mast ?

 

[G7RUX]

Yes there is, although you may be able to buy fibreglass tubes locally which are long enough.

 

[jazzboypro]

Yes there is, although you may be able to buy fibreglass tubes locally which are long enough.

Looks like the mast is the culprit. Since i retracted the metal portion of the mast the antenna is usable from 6 to 40. SWR is still to high on 80 (6.5). How well my signal will get out is another thing. I'll probably spend the evening trying to find out. Tomorrow i will try to source fiberglass poles locally if not can you suggest a brand of pole (preferably telescoping) and a good place to buy them ?

 

[G7RUX]

That makes a good bit of sense. You might find the 80m problem is related to the height of the antenna over the building.

Regarding suppliers, I’m not sure where you are so I might not be the best person to ask. However, fibreglass fishing poles are widely available and pretty cheap so might be a reasonable way to test the idea without spending a lot, plus the pole will be useful for a bit of portable operating later on!

 

[jazzboypro]

That makes a good bit of sense. You might find the 80m problem is related to the height of the antenna over the building.

Regarding suppliers, I’m not sure where you are so I might not be the best person to ask. However, fibreglass fishing poles are widely available and pretty cheap so might be a reasonable way to test the idea without spending a lot, plus the pole will be useful for a bit of portable operating later on!

I think this pole would suffice. it's the same size as the painter's pole i already have except that it's all fiberglass

 

[jazzboypro]

Finally received my fiberglass pole this morning. If time and weather allow i will installed it today if not than tomorrow

 

[jazzboypro]

Well, i spent most of my weekend trying to make this antenna work but to no avail. The fiberglass mast does not make any difference when compared to the mast that was there before. I tried to put the transformer on the mast itself and away from the mast but it does not make a significant difference. I try to put the mast a different heights and again no significant difference. Tried to attached the coax to the mast and also to let it hang from the transformer and again, no significant difference. At this point it would require an external tuner on a at least 5 bands. I suspect not much power would get at the antenna. Other than trying to lay the longer leg of the antenna differently i'm out of idea.