Modern transmitters and half-wave aerials: an interesting discussion

[Skywave]

Although primarily referring to transmitters, this article is also equally applicable to receivers. It attempts to address the misconceptions of the believed essential need to only use a half-wave dipole for a given frequency with a modern transmitter. This is a thorough and comprehensive article, so you will need time to read it in full - and digest it.
Generally speaking, I am in agreement with 99% of what is written - but others may disagree. Whatever, it certainly provokes some serious thinking about what is considered to be an 'effective' {TX / RX - feed-line - aerial} installation, especially when multi-freq. operation is desired.

http://www.qsl.net/arrlsb/Digest/Pages/A...nas03.html

Comments from all are welcomed (of course!)

Al. / Jan. 6, 2015. //

[pwdrive]

Thanks Al, I read through that, goes to show that the old timers knew their stuff (pre coax)

Lawrence.

[Skywave]

Thanks Al. I read through that: goes to show that the Old Timers knew their stuff (pre-coax)

Lawrence.

Indeed, Lawrence, indeed. My summary conclusions follow.

Now you and I well-know where, when and why co-ax is not only very useful, but when there is no practical alternative: "doing thing at VHF & up", for example. At lower freqs. - well maybe, just maybe. The recurring drift of that article seems to be "think in terms of balance": viz. a balanced aerial. Z mis-match? Adjust for conjugate reactance; twin-line can be arranged for very low-loss, hence VSWR not relevant.

That article certainly focussed my mind on the really relevant matters.


Al.

[pwdrive]

Didn't they use 300 ohm ribbon feeder in the States for TV antennas (reception) ?

Lawrence.

[Skywave]

Yes, Lawrence, that's my understanding.

It might just be worth mentioning that that type of line - its construction and materials used - is usually O.K. for reception purposes, but for the transmission of serious amounts of R.F. power, it can get quite lossy. A lot of the loss is caused by dirt etc. forming on the one-piece dielectric that separates the two wires. The type of line with gaps in it - and made with substantial insulating material - is to be preferred. Having said all that though, much research still indicates that balanced feeder using spacing bars - from 3 to 6 inches - still remains the best method of moving power R.F. from source to aerial load.

Aside.
From my perspective, all of this is highly relevant, since my new location looks like it could provide for an excellent balanced aerial and at a good height too, capable of operation at 3.5 MHz - or perhaps even lower.
But there will be a lot of work to do elsewhere before I can get back into 'radio stuff'.

Al.

[pwdrive]

Here's a calculator that lists line losses for some coax and some ribbon (wireman) for different lengths and frequencies, they give both dry and wet figures for ribbon, I don't know how comprehensive or accurate it is. There might be a better one on the web, I have not looked.

http://www.saarsham.net/coax.html

The new location sounds good Al, are you in yet?

Lawrence.

[Skywave]

Thanks, Lawrence.
Moved in yet? No: still packing. Should be in by the week-end though, all being well.

Al. / Wed., Jan 7th.

[Radio Fixer]

Well Al I struggled a little with the article and may have to read it again. I have never used a transmitter but as you say its also applicable to a receiver (certainly the theory).

What did I get from it: well I now know where the magic 50 Ohms comes from. And once upon a time I worked in military electronics and I can remember the vehicle whips having ATUs. I never got involved with them as I wasn't a transmitter guy but it gives me some understanding of what they were there for.

Recently, I restored an AK 308 and that has an aerial input that can be used called "Doublet" that I hadn't heard of before. I'm sure you know all about it but for those that don't please excuse me quoting a bit from the article written for the BVWS:

[i][i]Doublet (un-tuned dipole) is an American term, not heard by me before and AK sold a “D” Doublet Antenna Kit. The kit comprised coils of wire for the antenna leads and then spacers, to use on the wire, to create a parallel transmission line input to the house and radio set. Visualising this is easy if you think of the FM window aerials that most of us will have tried. Here, a piece of 300 Ohm balanced feeder is opened out at the ends to the desired lengths to make, in this case, a tuned aerial. To make the AK doublet antenna the dipole sections would have been mounted remote from the house, away from mains borne electrical interference, and then the feed in would have been balanced. To do this the kit supplied spacers, standoffs and importantly a dual lightening arrestor. In information given to me the makers say “… noise picked up by one of the two lead ins is balanced out by bucking it against identical noise in the other lead in.”

[/i][/i]

I also have a picture of the kit:
   

I could write more as I am using a Wellbrook mag aerial and very good it is even just stood on the window sill of the Den. The coax feed (50 Ohms of course) is just wired to the long wire input with no matching. I did have it suggested that I should include a resistor to pad the 50 Ohms out to a few hundred but that to me just seemed like loosing the power in the resistor rather than in the mismatch. Can standing waves be a problem on a broadcast set up? I think not but that's only a gut feeling.

The nice man at Wellbrook did say I could fit a Balun (back to the article again) but said it was probably worth only a couple of dBs.

May experiment when I get it rigged properly away from the house.

Phew! Long one for me

Gary