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1941 Packard 110 with original Motorola radio. Radio was recently restored and works perfectly. Not wanting to add any visible external antenna and, without it, the only sound I get are a couple of faint AM stations mostly drowned out by the "audio tachometer" that the radio has become without an antenna. All you hear is the ignition noise amplified.

Is there a way to plug an iphone into the AM antenna jack and just stream music through the Motorola radio? Electrical system is original 6V positive ground so want to avoid anything requiring a power hookup. Do I need to be concerned about "impedance" or anything else if there is a jack that adapts the iphone phono plug to the AM antenna jack on the radio?

No, not directly. The antenna input feeds into the first radio section where the over the air signal gets processed for all the different frequencies and variations in amplitude before a single signal gets selected and is sent on to the rest of the radio. The iPhone is already putting out a line signal of nothing but processed sound information and that output would need to go straight into the final audio section to be amplified and sent on to the speaker.

There are some options. One is send the radio back to the shop and have them add a line in jack for using the iPhone headphone output. You could alternately have them add a bluetooth module if you wanted to wirelessly stream from the iPhone. Some modules can accommodate both line in and bluetooth but the downside to modules is some may need changes to the radio chassis which are not reversible if someone ever wanted to have only the stock radio again.

Another option is to purchase something like the RediRadredirad.com which does plug into the radio antenna jack and only needs a power source. It has the circuitry to convert the iPhone line output back into RF and send it out on an unused frequency so it can be fed into the radio antenna jack. You then tune the radio to that RediRad frequency to play what is coming in from the iPhone. For your car you would want their AM only, positive ground unit. There may be something besides RediRad available but finding another that works on 6v positive ground may be the harder part.

If you bought a hidden antenna, it could also plug into the RediRad and give you the benefit of picking up the regular AM broadcast stations. When the RediRad line input is not being used the antenna signals would be passed straight thru and still be selected on the radio at their regular frequencies. Amazon has any number of small or completely hidden antennas that can be mounted in various out of the way spots but they are not as strong or sensitive as outside antennas. You also need to avoid placing them in locations entirely shielded by body metal so near glass is the best placement option.

Thank you, HH, for the quick and super comprehensive reply! I very much appreciate it. Your explanation is crystal clear and RedRad seems to be the very thing needed. I'll give them a call to find the right model.

I tried running an AM antenna under running board but all I get is still the "audio tach" despite following the original Packard shielding guidance from old service manuals and so had resigned myself to treating the radio as useless, despite it's restoration.

Many thanks again for the great help!

Curious what type antenna you tried under the running board. The small hidden type might not have worked very well with all the metal above it but am surprised a reasonably extended mast type didn't pick up more. If it was a mast type was the base held with some type of metal bracket so it could be grounded just as if it were mounted on a fender. If so, any chance the mast also became grounded in some way. Mast would need to be supported in a place or two but only with some plastic or other insulating material.

Your radio is analog and has a relatively weak amplifier compared to today's stds. The streaming music would be digital. Signal has to be converted with a device to communicate.
There are other add-ons for this that use a "hidden" antenna. If you keep the AM, your pretty much stuck. The "classic car stereos" come with all these modern features built-in. You need to upgrade the speakers as well.
I run my power antenna for "show" as the new contacts are not rated for the old motor amp draw so use a hidden antenna for the AM/FM. Some look like OEM faces, not sure about '41 Ps. There's a totally hidden under the seat remote controlled unit also, with separate audio. Popular with hot rods. Visit a car audio shop.
I suspect the motor noise is coming from the spark plug wires, and may benefit from a change of construction. There's other ways to suppress that, but I haven't dealt with that for decades. Some cars used metal shielding, like the corvette. Some used discs in the wheels, etc. Not sure how effective.
We're spoiled by our modern conveniences. I hear people wanting to add USB ports all the time to 6V+ cars. When you step back in time, you step back.

Good suggestions, or just mount a Blue Tooth speaker and stream from your phone. These speakers have good fidelity, but need to be recharged.

There are two types of interference noises - ignition and generator. Ignition can be cured with resistor wiring or resistor spark plugs, but don't do both. The generator noise can be dampened with a capacitor on the generator, coil and other locations. Also make sure your grounds are good. Getting a good antenna may help in getting rid of some of the interference noise.

Like Jim said: "When you step back in time, you step back."

Thanks, all, for keeping the discussion going. I used an extendable AM antenna (about 3 or 4 feet long), zip tied under the running board. I can't recall if or how I grounded it or whether the base was in contact with the metal structure or not. All I know is it made no difference from having no antenna plugged in at all.

As for the "audio tach" effect, I have replaced the generator with a 6V positive ground alternator (kept the generator on the shelf) for originality.

Do you guys know if I plug the RediRad unit in, will I still be dealing with the "audio tach" effect? The goal for me is simply to be able to turn on the original radio and hear sound coming from that 80 year old speaker with the warmth of those vacuum tubes driving it. But I don't want to drill a hold in the perfectly restored body to mount an antenna. Or, really, to have any visible antenna as I find all of them detract from the design of the car.

With the RediRad plugged into the radio the antenna jack will not be "floating" as it is now and picking up extraneous noise so the interference should be considerably diminished if not eliminated entirely -- at least while the IPhone is playing. What I cannot answer is if there is circuitry built into the RediRad that will prevent the open antenna socket on that item from picking up the noise when it is in pass thru mode. That would probably be a question to clarify but I think I remember something about no antenna connection needed in one of their descriptions.

You mentioned the radio came back from the shop working and in good condition so assume you had the antenna plugged into the radio for the test and maybe just laying on the bench?? For grins, you might pull your antenna from under the car and plug it into the radio with the antenna laying on the floor or seat (mast cannot be touching any body metal) and see if the noise is still an issue. If there is still noise possibly something is amiss either in the connection of the antenna plug or perhaps something in the lead in wire or radio has developed an issue.

If there is no noise with the antenna plugged in and laying somewhere and you can receive a few stations maybe try one of the small hidden non amplified so no extra power required AM/FM antennas and place it somewhere out of sight. If I understand properly, you don't really care much about receiving the broadcast stations so wouldn't be a big deal if it did not have a large gain and could not pickup very many stations.

If I was an electrical, not chemical, engineer, I probably could come up with some type of simple AM antenna, but I guess you're better off calling those redi people and ask them.
I'm familiar with autos for over 60 years but post war.
All you really need for an AM antenna is a length of wire so it could be "hidden" easily, if you know what to do. eg:news.ccrane.com/2018/04/09/how-to-make-a ... -powerful-am-loop-antenna-for-free/
Yes, good one JWL. They later put "condensers" aka capacitors on the generators for noise. I didn't mention that. I'm guessing somewhere someone has a post or video on reducing engine interference, but not sure. I would guess the proper shielding of the radio would eliminate that, but it's tubes, tubes generate heat so that has to be accounted for.

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An old fashioned wire coat hanger would work. Seen it used many times. Vintage fix.