Mag loop aerial for the Den

[Murphyv310]

Hi.
There might be some gains by tweaking the overall number of turns, I'd think that the bifilar winding could be a problem and give come capacitance effects possibly.
My Idea of reducing the secondary winding again "may" improve matching to the feeder.
Of course it's a "suck it and see" type of thing and unfortunately I wont be able to do any experimentation on mine as it's potted!
I will though experiment with all of this during the summer once my operation is over and I am back to normal

[Radio Fixer]

The best way for me would be to buy another 50 ft drum of coax and use it for testing. I could then wind different transformers and do the testing on the bench in the warm. Once I mount it up the garden in place of the Wellbrook this would be so awkward as to be impossible..

Guess the coax would get used up eventually and if not ....

Gary

[Radio Fixer]

Looking at coax there is cheap no name stuff and expensive Farnell and RS which is all branded.  Of course they could be all made in the same factory in China but one with quality control and the other none.  Anyone had bad coax?  Could it be the reason for the poor performance of my Wellbrook?  

Gary

[ppppenguin]

There is plenty of rubbish co-ax around. The stuff that we used to buy as "low loss" aerial downlead often had minimal braid cover. It leaked RF like a sieve and had very dubious low frequency characteristics. Of course solid dielectric co-ax isn't suitable for higher frequencies due to dielectric losses. Fine for video and low RF. For example I have loads of PSF1/3, a double screened 75R cable that was widely used for professional video. It would also be good for MW/LW but not quite so good at the upper end of SW. Not suitable for VHF and appalling for UHF.

Most of the stuff sold as satellite cable is electrically OK but the cheap varieties usually have an aluminium tape layer which will rot if any moisture gets in. Decent stuff like Webro WF100 has copper tape.

I'm less familiar with 50R varieties.

Gary, if you want some PSF1/3 I can let you have it at well below normal cost price. Let's say £5 for 100 feet. Some of us GVR folk are meeting up on Sunday and I think somebody's coming from Southend who might be willing to transport it.

http://www.canford.co.uk/Products/36-366...BBC-PSF1-3

[Radio Fixer]

Thanks Jeffrey but there are enough variables that I don't have a feel for and the Wellbrook is 50 Ohm so it would be better for me not to go against that.  Also the Home Brew design is 50 Ohm as well.

CPC do a named RG58 in 100M and I might go for that (anyone want to buy any or split it, coming to the NVCF ?)

   

They have another new one: http://cpc.farnell.com/concordia-technol...dp/CB18703

Not bad at £26 / 100m

Gary

[Murphyv310]

Hi.
I've found it very difficult to tell if one type of coax is actually better than another on Medium & Long waves.
Although a magic eye can give a rough indication of signal levels I would say an "S" meter is more accurate as is using a scope with a receiver with the AGC switched off.
I have actually used good quality audio screened cable over 5 or 6 meters of unknown impedance with no signal drop vs quality 50 ohm feeder.
Where you do find the difference is on an extended run 10 meters and more, then all cables due to loss and capacitance require proper matching.

As an aside I was greeted by a few US stations in the "X" band (1611-1700 Khz) on Monday and also Greenland on 570Khz, this was on the modded Wellbrook, switching to the frame aerial resulted in noise over these stations and no audible results.
US is difficult for me as I am in a built up area albeit at a good height, I have heard US MW stations before but not at this signal level.

[ppppenguin]

Just been down to the stores and found a drum of UR76. It says there are 2 lengths of 30m on the drum. This is very similar to RG58. It certainly has good braid coverage. You are welcome to one of the 30m lengths for £5. It's not on its original drum but I'm pretty sure I bought it from RS Components. Obviously I'll check that it still is 30m and that I haven't been cutting bits off it.

[Terry]

... Of course solid dielectric co-ax isn't suitable for higher frequencies due to dielectric losses ... Not suitable for VHF and appalling for UHF.

I know you've said this before, Jeffrey but, the next time you go down to the museum, have a look through my collection of odd coax cables (if Peter hasn't appropriated them all for flyleads!) and look for a sample of a very flexible cable marked 'RG59 HEC' (Head End Cable). If you inspect the end, you will see that it has a solid dielectric and I can assure you that it works very well at both VHF and UHF. I don't know if it is a special type of polyethylene, but it works! (Don't be fooled by its slight blue colour - this is the colour of the bonding agent between the aluminium shield and the dielectric!)

You may also find samples of a type with foam dielectric but it doesn't have the flexibility of the solid dielectric one.

Most of the stuff sold as satellite cable is electrically OK but the cheap varieties usually have an aluminium tape layer which will rot if any moisture gets in. Decent stuff like Webro WF100 has copper tape.

I would hope that no cable ever suffers moisture ingress! Whether the ali really does rot, I have no idea.

There is, however, another very good reason for avoiding this cable, usually sold as RG6.

The original CT100 and later WF/PF100 cable is manufactured to a strict specification, whereas RG6 is a cable type, NOT a specification!

Provided it follows the correct dimensions, it can have as much or as little braid coverage as you are prepared to pay for - provided you are buying it direct from the manufacturer. If buying retail, you won't have a clue - but it is unlikely to be a lot, hence the price differential between WF100 and RG6. Essentially, the RG6, alongside the cheap 'contract' aerials, is aimed at the cowboy fraternity. Avoid, avoid, avoid!

Not all RG6 is rubbish. Thousands of miles of it are used in this country for CATV drop cables but, it has lots of braid and - Jeffrey will like this - a flooding compound under the sheath - a kind of non-setting goo, which oozes to fill any splits in the sheath caused by cable damage to keep moisture out.

Beware if you come across any of this - a drum with a length of cable on it abandoned by a CATV installer because there isn't sufficient length for another installation, for example - it is not your lucky day! It is strictly for external use only as the tough PVC sheath produces poisonous cyanide gas in the event of a fire. In a CATV installation it is spliced onto an internal grade of cable at the entry point to the premises. It will be ideal for a run down the garden between the house and a shed but very little else!

[Radio Fixer]

Sounds good to me Jeffrey. It would be long enough to prove the coax run around the perimeter of the garden is OK. I may have to go by a more direct route but no problem and it would be fine for playing Home Brew on the bench.

Could you stick it in a large Jiffey bag and send it by My Hermes?

Then I could send you the all up money by PayPal (assume you have an account)

thanks Gary

[ppppenguin]

Turned out to be a 30m length and a 15m length on the drum. The 30m length will go by Royal Mail 2nd class small parcel for £2.80. I think I'd like to keep the 15m length. I'll pop a few BNCs in the package too.

Please PM me your address and I'll send my Paypal details.

As I said before I think one of the guys from Southend is coming to our meetup on Sunday so I could send it back with him if you prefer.

About to rush out so won't be able to deal with it until later or tomorrow.