Will it be ok, located here.

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ZL2BMH
Posts: 16
Joined: Mon Jul 16, 2018 8:49 am

Will it be ok, located here.

Post by ZL2BMH » Mon Jan 21, 2019 12:22 am

Hello All. ( I apologise for any simple mistakes in my typing. I am on very heavy prescribed medicine, which appears to play havoc with the grey matter. Sorry for this))

I have the SRP1a. I have bought a LNA4ALL (LNA) with the idea of capturing Meteor M2 and the new one, soon available in March. Plus to make an improvement on my NOAA pictures .

I would like to put the LNA which will be attached to a Turnstile or double crossed dipole (only one set of these crossed dipoles which is on the top of the short pole) on top of my 36ft mast, a really great place for it. However on top of the mast sitting on a rotator is a 5 element Yagi. Now, the idea is to clamp that short 3ft pole of the Turnstile which has the LNA strapped to it, to the top of the reflector element.

I am not good at describing a situation...... If you walk into my back yard you will see this big mast, set in concrete, look up and 36ft up you will see a enormous untrapped antenna, then on top of that, bolted to an element on the big Yagi antenna another little mast (3ft) with 2 crossed dipoles with elements about 14" long and a LNA strapped to it. That's as plain as I can make it, sorry.

Now I want to use my big Yagi antenna with the tiny weather satellite attached to it, to make a contact on HF putting 600 watts through this Yagi. Question is, will I damage the LNA.

I would really appreciate any answers I can get believe me.

Regards Jim.

snn47
Posts: 179
Joined: Tue Dec 27, 2016 11:00 pm

Re: Will it be ok, located here.

Post by snn47 » Mon Jan 21, 2019 1:08 am

How much of the 600 W or 57 dBm will be received by the turnstyle and is at the the input of the LNA requires a crystal ball.
Without knowing more about your setup e.g. are transmit frequency of the Yagi and receive frequency of the turnstyle close in frequency, what is polarisation of both antennas, I can only guess. For antennas beeing that close normal propagation models cannot be used to calculate the isolation between antennas.
Measured Isolation between antennas operated in the same frequency range beeing close is at best 20 dB.
If the frequency offset is sufficient you could apply filter with sufficient isolationto protect the LNA. Another approach could be a fast relais that will disconnect the LNA during transmission from the Turnstyle and switch it to a 50 Ohm load befor the ransmitter is keyed.

ZL2BMH
Posts: 16
Joined: Mon Jul 16, 2018 8:49 am

Re: Will it be ok, located here.

Post by ZL2BMH » Tue Jan 22, 2019 3:27 am

SNN47.

A excellent reply and a lot to think about which is not really a strong point of mine All I have ever done before retirement is man country police stations miles from anywhere. Virtually self taught by reading from books and a wonderful ham who did his best trying to teach me. I have never been involved in electronics unfortunately. But you have left me in no doubt that to get away with this is very chancey.

Sir, can you therefore answer a question the neighbour put to me and I'd like to know too. It's roughly 10 meters from the Turnstile sitting bolted to the reflector of my 5 ele. Yagi at the top of the mast it has not interfered with the SWR of the Yagi. Why not have the LNA actually inside the house? or for that matter under the floor sitting on the ground. Then send the remainder of the coax up through the floor which will be about another 4-5 meters.

Could you Sir, or anyone else for that matter, tell me if that would work?

You never stop learning as new challenges are thrown at you all the time in the great hobby of ours. But as the years are quickly running out for me, friends are departing at a too faster rate, we have to make the most of it while we can.

Regards to all Jim.

Regards,
Jim.

snn47
Posts: 179
Joined: Tue Dec 27, 2016 11:00 pm

Re: Will it be ok, located here.

Post by snn47 » Thu Jan 24, 2019 11:53 pm

The point of having a preamp as close to the antenna as possible, is to improve the overall noise factor of a receiving system. Any additional attenuation by a cable, (attenuation =length of cable times attenuation of cable type used in dB per meter found in the cable specs operating frequency) will reduce the power at the input of the LNA, but also reduce the noise factor.

If you can get hold of a milli watt meter you can meassure the power received when transmitting.

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