Remote antenna change

[VK4WDM]

I am going to put wire extension on my 10-80m Hustler trapped vertical so I can use it on 160m. Reports are that it works OK but it mucks up the tuning on 80m.

I am thinking about installing a small module with a latching relay on the 80m resonator so that I can disconnect and reconnect the wire as needed. I don't want to run any control wires up the antenna so I am thinking of using a 433MHz remote system. It would have to be low current draw because I want to use a small battery to power it at the top end.

Worthwhile idea or am I delusional? 

73

Wayne VK4WDM

[VK3ALB]

Hi Wayne,

I think it would be simpler to send DC up the coax to control the relay. No batteries required and bias tees are all you need.

[VK1JA]

Hi Wayne,

I'm keen to try this on my 6BTV too !

Have you thought about mounting an extra 80m coil (RM80S) up the top of the 6BTV? One is kept for use on 80m (with the stainless whip still attached) and the other is used for 160m that has the extended wire inserted into. They say that doing it this way you keep 80m, but of course you'd have to buy an additional RM80S and make up a bracket etc.

The added weight would mean you'd have to guy the 6BTV, if you haven't already.

Like this - https://www.youtube.com/watch?v=jsfhCiOtOn0

[VK4ADC]

Wayne

I use +12V up the coax to a "changeover" relay to switch between a vertical whip and a 10 metre long wire antenna (via an UN-UN) on my "house installation" {i.e. as different to my larger shack antenna install }.  

The bias tee at the bottom is simply a pair of coax sockets with the centres joined via a RF-style DC blocking capacitor and with the output DC fed via a RF choke of about 10mH with a bypass capacitor to 'ground/shield' on the output of the on/off switch - plus a series resistor and LED to show when the power is fed up to the relay.  All housed in a small jiffy box.. The DC for the bias tee / relay is sourced via a light fig-8 wire from the power supply for the transceiver.

The changeover relay box up top is a weatherproof ABS style containing a 10A 12V DPDT relay, another RF bypass capacitor, a diode wired across the relay coil, three stainless steel screws for output terminals and ground plus a SO239 for the coax connection in. It also houses the 9:1 UN-UN used to feed the longwire or whip, and in fact the DC feeds via the UN-UN (DC isolated secondary arrangement) before it hits the relay coil, diode and bypass capacitor. The otherwise unused relay contacts are used to 'ground' the unused antenna so that it acts as an additional counterpoise rather than as an active antenna via any relay contact-contact capacitance.

It all works very well and gives me some HF antenna flexibility with just one HF coax feed. So, yes, the bias tee idea really works.

[VK3LU]

Doug,
Is your RFC 10 millihenries or 10 microhenries?

Nev

[VK4ADC]

Doug,
Is your RFC 10 millihenries or 10 microhenries?

Nev

I used 10mH (well that was what it was marked)  - but a style suitable for RF, not audio - that I had on hand.  The value is not critical so long as XL > 500 ohms at the lowest frequency in use (ie 1.8MHz) & that calculates out as about 50uH (=565 ohms).

XL = 2piFL
     = 2 x 3.141592653 x 1.8 MHz x 50 uH
     = 5.7E+02 ohms (to 2 significant digits)
     = 565 ohms 

Use anything from 47uH/50uH upwards in an RF style when working at a 50 ohm impedance.

[VK3LU]

Doug,
I queried it because of an article I used to build a biasT. I found it on AD5X's site.
http://ad5x.com/images/Articles/BiasT3amp.pdf

Cheers
Nev

[VK4ADC]

The "rule of thumb" when selecting values for either bypass capacitors or isolation inductances is that the device must have a reactance greater than 10 times (or 1/10th for caps) the impedance to which it is connected.

That means that an isolation inductor has an inductive reactance > 10 times Zo, where Zo is 50 ohms when dealing with coax impedances of typically 50 ohms.

Conversely, bypass capacitor values must have a capacitive reactance < 10 times the circuit impedance to be effective. That means that the MAX value of capacitive reactance is 5 ohms at the lowest frequency involved. A better proposition to aim for is 1-2 ohms MAX.  A 47nF capacitor has a reactance of 1.9 ohms at 1.8MHz so that would be the smallest value you should consider using so that it will provide effective bypassing at RF.

XC = 1/(2piFC) = 1 / (2 x 3.141592653 x 1.8 MHz x 47 nF)
     = 1.88 ohms (to 3 significant digits)
     = 1.9 ohms 

Finally, remember that there are two inductors in parallel, one at each end, separated by coax  and the effective value of inductive reactance is the composite value - and lower than just having one in circuit, probably half if two identical values are used.

The AD5X article tells you what was found in that case with those values but not why they were selected in the first place.

[VK4WDM]

Hi Wayne,

I'm keen to try this on my 6BTV too !

Have you thought about mounting an extra 80m coil (RM80S) up the top of the 6BTV?  One is kept for use on 80m (with the stainless whip still attached) and the other is used for 160m that has the extended wire inserted into.  They say that doing it this way you keep 80m, but of course you'd have to buy an additional RM80S and make up a bracket etc.

The added weight would mean you'd have to guy the 6BTV, if you haven't already.

Like this - https://www.youtube.com/watch?v=jsfhCiOtOn0

Lou, Doug and Nev.  DC up the coax is the obvious way but not so exciting as squirting a bit of UHF RF at the relay

Jayson, I spotted that U tube last night. Easy way to go and resonators are not that expensive. (If anyone reading this has one for sale please drop me an email).

73

Wayne VK4WDM


I have only had the 6BTV for a couple of weeks and am quite impressed. Even on 80m where it is only a mobile resonator it does quite a good job - heard EU and NA on JT65.  I don't do anything serious on 160m but it will fun to have the band available

[VK1JA]

I think it would be a good way of doing it, retaining 80m and gaining 160m

If anyone else as a spare RM80S, I'll be interested too.