LF: Re: Antenna relays - multifilar Litz coil

[Markus Vester <markusvester@aol.com>]

> Marcus offered an interesting alternative, but the bifilar loading coil would need to have a low DC resistance (thus heavy wire).

My former loading coil was made of RF litzwire, 243 x 0.05 mm strands. These strands are grouped in 3 bundles, which can easily be separated and soldered individually. I haven't measured characteristic impedances of the resulting trifilar line, but I think it would be useful for DC as well HF as long as the differential voltage is not too large.

> I am not sure a (lossless) capacitor in parallel with the coil will lower the  antenna efficiency.

I think it would increase the coil losses, compared to a higher inductance coil of same Q, without the capacitor.

 Pcoil = I*U/Q

At same radiated power, you'll have the same voltage but higher coil current due to the extra capacitor.

 

All the best,

Markus (DF6NM)

-----Ursprüngliche Mitteilung----- 
Von: Rik Strobbe <rik.strobbe@fys.kuleuven.be>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Do, 19 Nov 2015 9:27 pm
Betreff: RE: Re[2]: LF: Re: Antenna relays

Hi Chris, 

 

your question is not easy at all, unless you want to spend  a lot of money. 

The easy (but expensive) solution would be to buy an appropiate vacuum relay.  

These can handle several 10kV and are RF-proof. 

As mentioned before for most other (and affordable) relays the contact voltages  

are given for DC or 50 (60) Hz. RF behaviour (even as low as 136kHz) will be  

different (worse). And RF voltages of several kV are very common on 136kHz. 

One option would be to put a number of relays "in series", but there are some  

pitfalls: 

- it will increase the (open) contact voltage, but not the contact-to-coil  

voltage (unless you will provide some kind of isolation in the relay driver) 

- you have to ensure that the voltage is equally divided over the relays: at  

least use identical relays, eventually equal (high voltage) resistors in  

parallel with each contact. 

 

Marcus offered an interesting alternative, but the bifilar loading coil would  

need to have a low DC resistance (thus heavy wire). 

 

73, Rik  ON7YD 

 

 

 

________________________________________ 

Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org]  

namens Chris Wilson [chris@chriswilson.tv] 

Verzonden: donderdag 19 november 2015 20:40 

Aan: Rik Strobbe 

Onderwerp: Re[2]: LF: Re: Antenna relays 

 

Hello Rik, 

 

Thursday, November 19, 2015 

 

 

Whilst you experts argue the technical niceties of the challenge, let 

me ask what might be a simple question to you guys. What is the 

minimum number of two contact vac relays I would need to go from 

strapped ladder line to top of loading coil, to unstrapped ladder 

line, disconnected from the top of the coil, and now connected to my 

coaxial feed line, presumably itself disconnected from ground and the 

bottom of the loading coil? It seems a lot would be needed to me.... 

:( Thanks! I can see me leaving wet weather gear by the front door and 

doing this manually! 

 

 

> Hello Stefan, 

 

> I am not sure a (lossless) capacitor in parallel with the coil will lower the  

antenna efficiency. 

> I would assume that it will affect the antenna impedance as seen 

> from the TX, but will not change the ratio Rrad/Rloss (what 

> determines the antenna effeciency). 

 

> 73, Rik  ON7YD 

 

 

 

 

 

 

-- 

Best regards, 

 Chris  2E0ILY                          mailto:chris@chriswilson.tv