- I'm interested in building a transmitting loop. As a consequence I'm interested in maximizing performance for transmitting as well as receiving. My target is a ~1/4 wave circumference loop constructed of 3/4" or greater soft copper tubing for 20M with some use on 30M as well.

Many (most) of the loops I've seen on the web have been coupled via a 1/5th diameter loop that's then directly connected to 50ohm coax. I've also seen a variation on that theme which resembles an autotransformer.

I've looked at a lot(!) of websites but haven't found the mathematical basis for this coupling method. What impedence does it actually yield and since the loop impedence must vary on either side of perfect resonance how is this reflected into the coupling loop? Are there any treatments on the makeup of the coupling coil w.r.t. performance?

Can anyone point me to a website or reference for study?

Always enjoy learning something new!

tnx

jim ab3cv

[Non-text portions of this message have been removed] - Jim Miller wrote:
> I'm interested in building a transmitting loop. As a consequence I'm

It looks like an ideal transformer with some shunt leakage

> interested in maximizing performance for transmitting as well as

> receiving. My target is a ~1/4 wave circumference loop constructed of

> 3/4" or greater soft copper tubing for 20M with some use on 30M as well.

>

> Many (most) of the loops I've seen on the web have been coupled via a

> 1/5th diameter loop that's then directly connected to 50ohm coax. I've

> also seen a variation on that theme which resembles an autotransformer.

>

> I've looked at a lot(!) of websites but haven't found the mathematical

> basis for this coupling method. What impedence does it actually yield

> and since the loop impedence must vary on either side of perfect

> resonance how is this reflected into the coupling loop? Are there any

> treatments on the makeup of the coupling coil w.r.t. performance?

>

> Can anyone point me to a website or reference for study?

>

> Always enjoy learning something new!

inductance. The transformation ratio is:

(turns ratio)*(flux in the larger loop)/(flux in the smaller loop). - Hi Jim,

I have used both methods of transforming the very low impedance of the

big loop to 50 ohms, and I can't say I find any difference; both work

well. Both methods yield a 50 ohm non-reactive impedance (SWR=1:1)at

the transmission line connection to either type transformer as

measured with an antenna analyzer. In either case, the tuning

capacitor and the sense loop are exquisitely sensitive to miniscule

changes, consistent with a high Q LC circuit. Using the 1/5 diameter

sense loop I generally end up making it too big and adjust the Z to 50

ohms by deforming the sense loop to something other than a perfect

circle, unlike most of the photos of perfect circles I have seen on

the Internet.

Dave WA6YSO

--- In loopantennas@yahoogroups.com, "Jim Miller" <jim@j...> wrote:

>

> I'm interested in building a transmitting loop. As a consequence I'm

interested in maximizing performance for transmitting as well as

receiving. My target is a ~1/4 wave circumference loop constructed of

3/4" or greater soft copper tubing for 20M with some use on 30M as well.

>

> Many (most) of the loops I've seen on the web have been coupled via

a 1/5th diameter loop that's then directly connected to 50ohm coax.

I've also seen a variation on that theme which resembles an

autotransformer.

>

> I've looked at a lot(!) of websites but haven't found the

mathematical basis for this coupling method. What impedence does it

actually yield and since the loop impedence must vary on either side

of perfect resonance how is this reflected into the coupling loop? Are

there any treatments on the makeup of the coupling coil w.r.t.

performance?

>

> Can anyone point me to a website or reference for study?

>

> Always enjoy learning something new!

>

> tnx

> jim ab3cv

>

>

>

> [Non-text portions of this message have been removed]

> - dave

thanks for the response. are there any implications on the conductor size of

the coupling element? do you just use some copper tubing or try to do the

shielded thing with a loop of coax?

i did some modeling with loopcalc for the frequencies i'm interested in and

to cover the cw band usually only takes 1-2pf change depending on band.

mechanical stability of the capacitor plates as well as overall loop

stability would seem to be important as a result especially if the loop is

mounted outside as i plan.

73

jim ab3cv - Jim, the resistance of the sense loop will be 50 ohms. So it does not

have to have a large surface area like the main loop, whose resistance

due to radiation is below 0.5 ohms and hence must be capable of

passing high current. I generally use #8 to #14 bare copper wire. I

have never run more than 50W, usually about 30W (20 meter psk).

According to a previous post, the MFJ super loop, rated for (I

believe) 100W, uses #14 copper wire for its sense loop. What size

wire would you use for a dipole? The same size will work for the

sense loop, though it must be sturdy enough to be self supporting. I

have never attempted to build a shielded sense loop out of coax.

Regarding the capacitor. From most ideal to least ideal but usable

(low to higher resistive losses) are vacuum variable, butterfly then

split stator. For transmitting application you should avoid

capacitors with a wiper due to resistive losses.

A commercially built capacitor have sufficient stability. Its

drive usually includes reduction gears. Being somewhat mechanically

challenged, I find the remote motor drive setup to be the most

difficult part of building small transmitting loops. Clearly

mechanical stability is a strong requirement.

Try to mount the capacitor close to the ends of the loop with soldered

connections between the capacitor terminals and the copper loop pipe.

I agree with your conclusion regarding mechanical stability. These

small loops are very reminiscent of plumbing.

Dave WA6YSO

--- In loopantennas@yahoogroups.com, "Jim Miller" <jim@j...> wrote:

>

> dave

>

> thanks for the response. are there any implications on the conductor

size of

> the coupling element? do you just use some copper tubing or try to

do the

> shielded thing with a loop of coax?

>

> i did some modeling with loopcalc for the frequencies i'm interested

in and

> to cover the cw band usually only takes 1-2pf change depending on band.

> mechanical stability of the capacitor plates as well as overall loop

> stability would seem to be important as a result especially if the

loop is

> mounted outside as i plan.

>

> 73

>

> jim ab3cv

>