Combining two different style antennas in to one signal.

[rwgast_lowlevellogicdesin]

Im not a part of any HAM forums, being new to SDR im not sure of any good forums for these type of questions. I figured there are plenty of people here that are in to RF though.

Im building a 3ft discone for reception down tords 75mhz, I plan to mount it on my roof along with a preamp, then run the coax in to my house. I have seen commercial discones with HF antennas mounted on the top and im curious how you do this? I would also like to build a smaller higher frequency discone and merge its signal in to the same path. I believe this is called a diversity array?

In the end preamps are expensive, so I am looking to try to cover the whole RTL SDR band with a few antennas that run in to one preamp then in to a single feedline off the roof in to the house. Is this possible?

Also as a side question, if you have a good preamp will you see any benefit from using an active antenna?

[babysitter]

Putting 2 or more antennas signals thru one cable is normally done by diplex filters, which consist of two or more band-pass filters. Basically they only prevent that a signal which is off-range for one of the antennas doesnt go there. Comparable to the crossover networks in your stereo, directing bass to the subwoofer and higher frequencies to the high-band speakers.

You might find some interesting parts in the cable internet access products, I could imagine a wall plate with a TV port (47 MHz and up) and a Data upstream port (maybe up to just below 47 MHz, read specs, could be dependent on your region) might match your requirement of splitting between your vhf discone and a shortwave antenna.

If you dont consider designing your own diplexer, have a look at RF relais. You get them ready made as satellite tv accessories.

Using a preamp at some other place than right at the output of the antenna is quasi forbidden, e.g. behind a cable length or a relay. This will give quite a blow to the performance. If you really want to do it, run shortest possible cabling from the antenna to the relay and from the relay to the amplifier.