Re: [IRCA] More coax experiments

[Rick Kunath <k9ao@xxxxxxxxxxx>]

Patrick Martin wrote:
> I thought that is what you would say. hi. A metal box eh? I guess I
> should look for one. I have used the plastic set ups since 1988 and I
> still have heard a lot of great DX. I wonder going to all of this work
> is going to really make any difference in what I hear? I know lab tests
> and the like do show a difference, but in the real world will be do
> anything?  I guess I will try it for the Eastern beverage as I am more
> concerned about the directivity of that more than the EWEs. The EWEs
> pretty much have the directional pattern they are supposed to have, even
> without all of the better coax and a metal box. I can try the Eastern
> beverage. If I notice a difference, I can always change the SW EWE set
> up. The vertical I don't care much about as I use it mainly for SW. The
> WNW EWE has more buried coax and will be harder to change out the coax
> there. 
>    Maybe I can find a small metal box at Radio Shack tomorrow. I can
> install a coax fitting on the one side and then a couple banana jax on
> the other. Then everything will be shielded The banana jax will be
> inside the metal box as well as the coax fitting. At least I can check
> out the coax better terminating the end with a cap. That way I can tell
> how much the coax is leaking. At least it is a start.

I think if it were easy, the first thing I'd try would be to replace the 
coaxial cable with something better shielded and do a receive test to 
see what happens with what you have. If the coax you already have has 
marginal shielding, changing the box to a metal one likely won't matter, 
as there are plenty of places along the run of cable where signals can 
leak back in.

Once the cable has been changed, adding a sleeve-type choke at the 
antenna end would likely be my next step. This will choke off any 
currents coming back along the length of the coax from making their way 
from the outside of the shield and onto the inside of the shield at the 
antenna end and on their way back down the (now properly shielded new) 
coax to your receiver.

Once that's done, if you needed more isolation, I'd replace the box with 
a metal one. If the sleeve-type choke works well enough, use the plastic 
box you have.

A sleeve-type choke is a series of ferrite cores, sized to slide snugly 
over the coaxial cable. These are typically a series of ferrite cores 
stacked up maybe a foot or more long at medium wave. You need enough 
cores to get a high impedance to the RF at the frequency of interest. 
There are commercial chokes available, or you can make up your own with 
purchased cores (Amidon comes to mind, and they'll help you pick out the 
right cores), and seal the outside. I usually make these up separate 
from the actual coax run to the antenna. That way you can remove them 
and move them around. I put a connector at each end and place them in 
series with the cable run at the antenna end.

Some folks use toroid cores and wind the coax through the core to make a 
choke. I don't like these as the inter-turn capacity negates some of the 
isolation effects. But for testing, if you have a core handy, at least 
you could see what the partial effect would be.

If you have a signal generator or some source of RF at medium wave, you 
can do a pretty good test for cable leakage by just laying it out on the 
ground straight, applying RF to one end, and a dummy load to the other, 
and moving a radio with a ferrite rod antenna along the cable to see if 
yo have any hot spots. You don't need a dummy load of any power handling 
capacity, as the RF levels for testing like this, or terminating the 
coax for a receiver-connected leakage test are very low level.

You could make yourself up a dummy load just with a metal box with a 
coax connector and a carbon resistor inside (equal to the cable's 
characteristic impedance) that would work for testing. But, low cost 
commercial units made up inside a connector body are pretty cheap, 
likely cheaper than the parts to make one up.

Rick Kunath




> Thanks again.
> 
> 73,
> 
> Patrick
>  
> 
> Patrick Martin
> KAVT Reception Manager
> 
> _______________________________________________
> IRCA mailing list
> IRCA@xxxxxxxxxxxxxxxx
> http://montreal.kotalampi.com/mailman/listinfo/irca
> 
> Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers
> 
> For more information: http://www.ircaonline.org
> 
> To Post a message: irca@xxxxxxxxxxxxxxxx
> 
> 

_______________________________________________
IRCA mailing list
IRCA@xxxxxxxxxxxxxxxx
http://montreal.kotalampi.com/mailman/listinfo/irca

Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers

For more information: http://www.ircaonline.org

To Post a message: irca@xxxxxxxxxxxxxxxx